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Description  |
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CROSS-REFERENCE TO RELATED APPLICATIONS
The algae/support composite employed in the present invention is generally
described and claimed in copending and commonly assigned application Ser.
No. 07/026,927, entitled IMMOBILIZED BLUE-GREEN ALGAE and filed of even
date in the name of Ronald S. Nohr. The algae/support composite in sheet
form employed in the present invention is generally described and claimed
in copending and commonly assigned application Ser. No. 07/027,282,
entitled IMMOBILIZED BLUE-GREEN ALGAE IN SHEET FORM and filed of even date
in the names of J. Gavin MacDonald, Ronald S. Nohr, and William E.
Maycock.
BACKGROUND OF THE INVENTION
The present invention relates to immobilized blue-green algae having
enhanced growth and nitrogen-fixation rates. More particularly, the
present invention relates to a multilayered structure incorporating such
immobilized blue-green algae therein.
In recent years, there has been an increasing interest in the use of
biological nitrogen fixation as a replacement for chemical nitrogen
fertilizers. However, biological nitrogen fixation can be carried out only
by a limited number of microorganisms. Foremost, perhaps, among such
microorganisms are the blue-green algae, although not all blue-green algae
are capable of fixing nitrogen. Such algae, i.e., the nitrogen-fixing
blue-green algae, are able to fix nitrogen in an aerobic environment.
Moreover, they are photosynthetic. For examples of references dealing
generally with nitrogen fixation by blue-green algae, see H. W. Paerl,
Can. J. Bot., 60, 2542 (1982); J. L. Ramos and M. G. Guerrero, Arch.
Microbiol., 136, 81 (1983); J. S. Chapman and J. C. Meeks, J. Bacteriol.,
156, 122 (1983); O. Ito and I. Watanabe, New Phytol., 95, 647 (1983); W.
A. Wurtsbaugh and A. J. Horne, Can. J. Fish. Aquat. Sci., 40, 1419 (1983);
P. M. Mullineaux et al., J. Gen. Microbiol. 129, 1689 (1983); Y. Chen,
Zhiwu Shenglixue Tongxun 1983, 22; P. S. Tang et al., in C. K. Tseng,
Editor, "Proceedings of the Joint China-U.S. Phycology Symposium," Science
Press, Beijing, China, 1983, pp. 339-63; L. Leonardson, Oecologia. 63, 398
(1984); R. G. Elder and M. Parker, J. Phycol., 20, 296 (1984); L. J. Stal
et al., Marine Biology, 82, 217 (1984); B. Bergman et al., Z.
Pflanzenphysiol., 113, 451 (1984); and D. H. Turpin et al., Plant
Physiol., 74, 701 (1984).
Under conditions of nitrogen deficiency, some of the vegetative cells of
the algae differentiate into heterocysts which are capable of fixing
atmospheric nitrogen. See, by way of illustration only, A. Kumar et al.,
J. Bacteriol., 155, 493 (1983); M. Roussard-Jacquemin, Can. J. Microbiol.,
29, 1564 (1983); and references cited therein.
The use of blue-green algae as a biological nitrogen fertilizer is, of
course, known. Such algae have been studied for or used in the cultivation
of rice; see, e.g., G. S. Vankataraman and S. Neelakantan, J. Gen. Appl.
Microbiol., 13, 53 (1967); W. D. P. Stewart et al., in "Nitrogen and
Rice," International Rice Research Institute, Los Banos, Laguna,
Philippines, 1979, pp. 263-85; G. S. Venkataraman in "Nitrogen and Rice,"
International Rice Research Institute, Los Banos, Laguna, Philippines,
1979, pp. 311-21; A. Agarwal, Nature., 279, 181 (1979); O. Ito and I.
Watanabe, Soil Sci. Plant Nutr., 27, 169 (1981); G. S. Venkataraman,
Current Science., 50, 253 (1981); G. S. Venkataraman, Trans. Int. Congr.
Soil Sci. 12th, 2, 69 (1982); Z. T. Begum, Bangladesh J. Bot., 12, 127
(1983); L. Shanghao (S. H. Ley) and W. Qianlin, in C. K. Tseng, Editor,
"Proceedings of the Joint China-U.S. Phycology Symposium," Science Press,
Beijinq, China, 1983, pp. 479-96; A. Islam et al., Indian J. Agric. Sci.,
54, 1056 (1984); V. Rajaramamohan Rao and J. L. N. Rao, Plant and Soil ,
81, 111 (1984); B. S. Kundu and A. C. Gaur, Plant and Soil., 81, 227
(1984); I. Watanabe, Outlook on Agriculture, 13, pages unknown (1984); and
H. C. Bold and M. J. Wynne, "Introduction to the Algae," Second Edition,
Prentice-Hall, Inc., Englewood Cliffs, N.J., 1985, p. 37.
In addition, at least one company is marketing a blue-green algal
fertilizer for the lawn and garden market. The fertilizer is prepared by
blending dried algae with a soil-like carrier which allegedly prevents the
dormant algae from dying. See B. R. Schlender, "New Uses for Algae Improve
Image of a Lowly Plant Group," The Wall Street Journal, Friday, July 11,
1986, p. 25; Biotechnology News, Oct. 9, 1985, p. 6; and E. Eckholm,
"Science Tries to Harness Bacterial Overachievers," The New York Times,
Tuesday, Feb. 24, 1987, p. 15. Moreover, the application of blue-green
algae to tomato plants reportedly resulted in 45% more growth (weight
gain) than plants treated with the same amount of commercial fertilizer.
It was postulated that the weight gain may be due to the secretion of a
plant-growth hormone (Science/Technology Concentrates, Chemical and
Engineering News, date unknown). Interestingly, cultivated soils
apparently contain inconsequential numbers of blue-green algae; see W.
Zimmerman et al., Soil Science, 130, 11 (1980).
In contrast with the large number of studies on the fixation of nitrogen by
blue-green algae and the use of blue-green algae as a nitrogen fertilizer,
little work apparently has been done with immobilized blue-green algae.
Moreover, what work has been carried out was done primarily with entrapped
cells, as described in the paragraphs which follow.
According to A. Muallem, Biotech 83: Proc. Int. Conf. Commer. Appl. Implic.
Biotechnol. 1st 1983, pp. 1037-50, seven species of cyanobacteria, or
blue-green algae, were entrapped in polyurethane foams. The entrapped
cells, however, were used for the long-term photoproduction of hydrogen
and NADPH.sub.2 from ascorbate and water. To immobilize the algal cells,
pieces of foam were added to the culture vessels before autoclaving and
inoculating with the algae. One species did not remain entrapped in any
foam. In at least some cases, freeze-thaw cycles were employed as part of
the immobilization procedure. The reference includes one electron
micrograph, regarding which it was reported that short filaments and
single cells were seen adhering to the polyurethane fibers which
constitute the pore walls, and that cell envelope components were solely
responsible for cell hydrophobicity which plays the major role in adhesion
of benthic cyanobacteria on solid surfaces which have little or no surface
charge. It is clear that no effort was made to measure nitrogen fixation
by the immobilized algae. The immobilized cells apparently did not grow
since they did not undergo any regenerative carbon metabolism.
Whole filaments of a nitrogen-fixing cyanobacterium were immobilized by
entrapment in calcium alginate gel beads; S. C. Musgrave et al.,
Biotechnol. Lett., 4, 647 (1982) and Adv. Ferment. Proc. Conf. 1983, pp.
184-90. The immobilized cyanobacterium were used in various
continuous-flow reactors for the sustained production of ammonia.
Finally, protein turnover in immobilized cells of a cyanobacterium was
studied by M. Potts, J. Bacteriol., 164, 1025 (1985). The cells were
immobilized by transfering a sample of a cell suspension to Whatman 3MM
filter discs (23-mm diameter) which were supported on steel pins. The
study employed only 50-microliter samples of a well-dispersed cell
suspension and the algal filaments reportedly were immobilized immediately
within the confines of the upper matrix of the support, occupying a
circular area approximately 8 mm in diameter. The immobilization appears
to be primarily an entrapment, and nitrogen fixation by the cells was not
studied.
While blue-green algae were not involved, it perhaps should be mentioned
that the continuous production of ammonia by an immobilized
nitrogen-fixing-system depressed mutant strain of a bacterium, Klebsiella
pneumoniae, has been reported; K. Venkatasubramanian and Y. Toda,
Biotechnology and Bioengineering Symp. No. 10, 237-45 (1980).
Immobilization involved mixing the cells with a collagen dispersion,
adjusting the pH, casting a membrane, and crosslinking it with a mild
solution of glutaraldehyde. Thus, the cells were entrapped in a collagen
membrane.
SUMMARY OF THE INVENTION
It now has been discovered, quite unexpectedly, that certain
nitrogen-fixing blue-green algae, when immobilized, demonstrate
significantly enhanced growth and nitrogen-fixation rates compared to
algae growing in suspension, and that when organized in the form of a
multilayered sheet-like structure, such immobilized blue-green algae are
especially useful in a variety of agricultural applications.
Accordingly, the present invention provides a sheet-like, water-pervious,
nutrient-producing, multilayered structure having a thickness which is
substantially less than either its breadth or width, which structure
comprises:
A. a first layer having a first surface and a second surface which
comprises a composite consisting essentially of a substantially
water-insoluble support having a surface energy of at least about 19 dynes
per cm to which nitrogen-fixing filamentous heterocystous blue-green algae
are attached, said support being substantially free of substances which
have a significant deleterious effect on the viability of the immobilized
algae;
B. a second layer adjacent to and contiguous with at least a portion of
said first surface of said first layer and attached to said first surface
of said first layer in such a manner as to substantially maintain said
second layer adjacent to and contiguous with said first surface of said
first layer; and
C. a third layer adjacent to and contiguous with at least a portion of said
second surface of said first layer and attached to said second surface of
said first layer in such a manner as to substantially maintain said third
layer adjacent to and contiguous with said second surface of said first
layer;
wherein at least one of said second layer and said third layer is at least
partially transparent to actinic radiation.
In certain preferred embodiments, the blue-green algae are attached to the
support by means of the algal heterocyst cells.
In other preferred embodiments, the support is a cellulosic, such as a wood
pulp.
In yet other preferred embodiments, the structure has a plurality of
raised, three-dimensional shapes over at least a portion of at least one
surface; the structure has at least one opening therethrough; or the
structure has a generally nonplanar configuration
The structure of the present invention is useful as a nutrient-producing
source for agricultural applications. Such applications include commercial
farming, truck farms, greenhouses, home gardening, outdoor and indoor
landscaping, and the like.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are perspective view representations of sheet-like
structures which illustrate two embodiments of the present invention.
FIG. 2 is a perspective view representation of a sheet-like structure of
the present invention which has a plurality of raised, three-dimensional
shapes over at least a portion of at least one surface.
FIG. 3 is a cross-sectional view through line 2--2 of FIG. 2 illustrating
one embodiment of the raised shapes shown in FIG. 2.
FIGS. 4A and 4B are perspective view representations of two variations of
the structure of FIG. 5, adapted for use with a single plant which can be
either outdoors or indoors.
FIG. 5 is a perspective view representation of a sheet-like structure of
the present invention which has a plurality of openings therethrough.
FIGS. 6, 7A, and 7B, inclusive, are perspective view representations of
sheet-like structures of the present invention, each of which has a
generally nonplanar configuration.
FIG. 8 is a plot of algal growth on each of four different supports and in
a control culture, all in a liquid medium, against time.
FIG. 9 is a plot of algal growth on each of three different supports and in
a control culture, all on agar plates, against time.
FIGS. 10-24, inclusive, are hand-drawn representations of scanning electron
micrographs at varying degrees of magnification of blue-green algae
immobilized on or attached to a variety of supports.
DETAILED DESCRIPTION OF THE INVENTION
Although the term "immobilized" has been used rather broadly in the prior
art, such term is used herein more narrowly. As applied to a composite
useful in the present invention, the term is meant to be essentially
synonymous with "attached" and is intended to exclude entrapped algal
cells. However, the nature of the attachment is not critical, although the
algae preferably are attached by means of the heterocyst cells.
As already stated, the present invention provides a sheetlike,
water-pervious, nutrient-producing, multilayered structure having a
thickness which is substantially less than either its breadth or width.
The term "sheet-like" is used herein to mean only that the structure
itself, but not necessarily the volume occupied by it, has a thickness
which is substantially less than either the breadth or width of the
structure. That is, the structure need not be planar.
To be water-pervious, it is only necessary for the structure to permit
water to pass from one surface thereof to the other. The means by which
this is accomplished is not important, provided that the structure does
not block the passage of the nutrients, such as soluble nitrogen
compounds, through the structure. For example, water can pass through the
structure by capillary action or by simply flowing through sufficiently
large pores or openings in the structure. If water cannot pass through the
structure as described, the structure is, for the purposes of the present
invention, water-impervious.
The structure is nutrient-producing because of the algae immobilized on the
support which comprises the first layer. The algae are known to produce
soluble nitrogen compounds from elemental nitrogen present in the
atmosphere. However, the algae very likely excrete other nutrients into
their surrounding environment, some or all of which may be utilizable by
plants in reasonable proximity to them. Thus, the term
"nutrient-producing" is meant to include all of such nitrogen compounds or
other nutrients, although the soluble nitrogen compounds probably are the
most significant.
The first layer of the multilayer structure of the present invention
comprises a composite consisting essentially of a substantially
water-insoluble support having a surface energy of at least about 19 dynes
per cm to which nitrogen-fixing filamentous heterocystous blue-green algae
are attached, said support being substantially free of substances which
have a significant deleterious effect on the viability of the immobilized
algae. Preferably, the algae are attached to the support by means of the
algal heterocyst cells.
As used herein with reference to nitrogen-fixing filamentous heterocystous
blue-green algae, the term "viable" or a variation thereof means simply
that the growth and nitrogen-fixing characteristics of the immobilized
algae are substantially unimpaired when compared to such characteristics
of the same algae in suspension.
In general, the use of any nitrogen-fixing filamentous heterocystous
blue-green algae comes within the spirit and scope of the present
invention. Such algae are chlorophyllous prokaryotic organisms which can
be classified by either of two systems. The first system is a taxonomic
classification described by T. V. Desikachary in a chapter entitled
"Classical Taxonomy," in N. G. Carr and B. A. Whitton, Editors, "The
Biology of Blue-Green Algae," University of California Press, Berkeley,
1973, pp. 473-481. According to this system, the nitrogen-fixing
filamentous heterocystous blue-green algae in general are those which fall
within the following families and genera:
______________________________________
Order Family Genus
______________________________________
Nostocales Oscillatoriaceae
Arthrospira
Borzia
Crinalium
Gomontiteilla
Isocystis
Lyngbya
Microcoleus
Oscillatoria
Phormidium
Schizothrix
Spirulina
Symploca
Nostocaceace Anabeana
Anabaenopsis
Aphanizomenon
Aulosira
Camptylonemopsis
Cylindrospermum
Hormothamnion
Nostoc
Raphidiopsis
Richelia
Wollea
Scytonemataceae
Coleodesmium
Hydrocoryne
Scytonema
Scytonematopsis
Tolypothrix
Microchaetaceae
Fortiea
Michrochaete
Rivulariaceae Calothrix
Dichothrix
Gloeothrichia
Hammatoidea
Homoeothrix
Kyrtuthrix
Rivularia
Stigonematales
Capsosiraceae Capsosira
Hyphomorpha
Loriella
Pulvinularia
Stauromatonema
Nostochopsidaceae
Mastigocoleus
Nostochopsis
Mastigocladaceae
Brachytrichia
Iyengariella
Mastigocladus
Stigonemataceae
Doliocatella
Fischerella
Geitleria
Hapalosiphon
Schmidleinema
Stigonema
Westiella
Westiellopsis
______________________________________
The second classification system is that described by R. Rippka et al., in
"Generic Assignments, Strain Histories and Properties of Pure Cultures of
Cyanobacteria," Journal of General Microbiology, 111, 1-61 (1979). In that
system, all of the nitrogen-fixing filamentous heterocystous species are
members of Sections IV and V and comprise species in the following genera:
______________________________________
Section Genus
______________________________________
IV Anabeana
Nodularia
Cylindrospermum
Nostoc
Scytonema
Calothrix
V Chlorogloeopsis
Fischerella
______________________________________
The above-described classification systems are given by way of illustration
only, however, and are not to be construed as in any way limiting either
the spirit or the scope of the present invention. That is, any filamentous
heterocystous blue-green algae which is capable of attaching to or being
immobilized on a support and which fixes nitrogen in such attached or
immobilized state is deemed to come within the scope of the present
invention, whether or not such algae fit either or both of the foregoing
classification systems. Thus, such algae include unclassified algae, algae
which do not fit either of the foregoing classification systems, algae
which are mutants of known algae, algae which have been subjected to gene
manipulation procedures, and the like. Moreover, the term "filamentous
heterocystous blue-green algae" also is deemed to include organisms which,
while perhaps not properly classified as algae, exhibit the
nitrogen-fixing and support-attaching characteristics described herein,
such as may be derived through gene splicing and/or cell fusion
techniques.
The nitrogen-fixing filamentous heterocystous blue-green algae grow as
chains of cells or filaments. Generally, the filaments are composed of
uniform small cells, known as vegetative cells, which grow an divide
within the filament. As the length of each filament increases, it becomes
more susceptible to shear forces in the environment which cause the
filament to break. Each fragment then continues to grow independently. The
average filament length thus tends to remain relatively constant as the
total number of cells increases.
When cells are grown under conditions which induce nitrogen fixation, i.e.,
growth in a nitrogen-limited environment, some of the vegetative cells in
the filament differentiate to form larger, specialized cells, called
heterocysts. The heterocysts are known to be the actual sites of nitrogen
fixation. On the average, about one in every 20 to 50 cells develops into
a heterocyst. The heterocysts produce fixed nitrogen, i.e., one or more
water-soluble nitrogen-containing compounds which can be utilized by
plants, which is transferred to the vegetative cells, while the vegetative
cells in turn supply energy and carbon compounds to the heterocysts.
Excess fixed nitrogen produced by the heterocysts is secreted from the
filament, although it is not known whether the actual site of secretion is
the heterocyst or the adjacent vegetative cells.
In general, the substantially water-insoluble support to which the
nitrogen-fixing filamentous heterocystous blue-green algae are attached to
give a composite useful in the present invention can be any material which
has a surface energy of at least about 19 dynes per cm. As will be shown
hereinafter, such algae will attach, although to a relatively small
extent, to supports having a surface energy as low as about 19 dynes per
cm. In fact, such algae have been observed to attach to supports having
surface energies from about 19 to around 115-120 dynes per cm. However,
the extent of attachment increases when supports having higher surface
energies are employed.
Because the structure of the present invention is intended for agricultural
applications, a support surface energy of from about 30 to about 115 dynes
per cm is preferred in order to provide a structure which is reasonably
efficient in the production of nutrients, e.g., soluble nitrogen
compounds. Most preferably, the support will have a surface energy of from
about 40 to about 115 dynes per cm.
As already stated, the support should be substantially free of substances
having a significant deleterious effect on the viability of the attached
algae. Because of the wide variety of supports which can be employed in
the present invention, it is not practical to list such substances which
may be either naturally occuring in the support or added as a result of
manufacturing or processing requirements. However, those having ordinary
skill in the art will know of substances and conditions which are harmful
to algae, such as extremes in pH, chemicals which are toxic to the algae,
and the like. For example, unprocessed pine particles or fibers will kill
the nitrogen-fixing filamentous heterocystous blue-green algae. However,
if the wood fibers are processed to remove the deleterious substances,
such as during the preparation of a thermomechanical wood pulp, the algae
will attach to them. Another example of supports having deleterious
substances associated therewith are various woven and nonwoven fabrics
which have either in or on the fibers various compounds employed as sizes,
lubricants, preservatives, and the like, many of which are extremely
difficult to remove from the fabric.
Thus, the support can be a natural, modified natural, or synthetic
material. The support can be discontinuous or continuous and can be any
shape. The term "discontinuous" is used herein to mean that the support is
of such a size that a quantity of the support generally must be employed
to form the first layer. Examples of discontinuous supports are particles,
granules, fibers, and the like. Alternatively, the structure can be
continuous or sheet-like. If continuous, the support can be sheet-like ab
initio, in which case the algae are allowed to attach directly to the
layer. If discontinuous, the support can be employed as the first layer as
is or formed into a continuous or sheet-like structure. Moreover, the
support can be porous or nonporous.
If continuous, the support can be drapeable, flexible, or even rigid. The
support can be film-like by having an absence of visually perceptible
pores. Alternatively, the support can have a rather loose or open
construction, such as that which might be present in a woven or knitted
fabric or a nonwoven web. Indeed, the support can be a woven, knitted, or
nonwoven fabric or web.
Examples of suitable supports include, among others,
poly(tetrafluoroethylene), polymers and copolymers of ethylenic monomers
having less than four fluorine atoms per monomer, glass, polyolefins,
polyolefins having chemically modified surfaces, polyesters, polyamides,
and cellulosics.
For the purposes of the present disclosure, the term "polyolefin" is meant
to include any polymeric material a major constituent of which, i.e., at
least 50 percent by weight, is a polyolefin. Thus, the term includes
homopolymers, copolymers, and polymer blends.
Copolymers can be random or block copolymers of two or more polyolefins (or
of two or more different polyolefin monomeric precursers) or of one or
more polyolefins and one or more nonpolyolefin polymers. Similarly,
polymer blends can utilize two or more polyolefins or one or more
polyolefins and one or more nonpolyolefin polymers. As a practical matter,
homopolymers and copolymers and polymer blends involving only polyolefins
are preferred, with homopolymers being most preferred.
Examples of polyolefins include polyethylene, polystyrene, poly(vinyl
chloride), poly(vinyl acetate), poly(vinylidene chloride), poly(acrylic
acid), poly(methacrylic acid), poly(methyl methacrylate), poly(ethyl
acrylate), polyacrylamide, polyacrylonitrile, polypropylene,
poly(1-butene), poly(2-butene), poly(1-pentene), poly(2-pentene),
poly(3-methyl-1-pentene), poly(4-methyl-1-pentene),
1,2-poly-1,3-butadiene, 1,4-poly-1,3-butadiene, polyisoprene,
polychloroprene, and the like.
The preferred polyolefins are those prepared from unsaturated hydrocarbon
monomers, with polyethylene and polypropylene being most preferred.
As used herein, the term "cellulosic" is meant to include any material a
major constituent of which, i.e., at least 50 percent by weight, is
cellulose or a cellulose derivative. Thus, the term includes cotton,
cellulose acetate, cellulose triacetate, rayon, thermomechanical wood
pulp, chemical wood pulp, debonded chemical wood pulp, milkweed, and the
like. The preferred cellulosics are the various wood pulps, the
preparations of which are well known to those having ordinary skill in the
art.
The preferred support materials are polyolefins, surface-modified
polyolefins, and cellulosics, with the polyolefins and cellulosics being
more preferred. The most preferred support materials are cellulosics.
As already stated, it is necessary that the support be substantially free
of substances having a significant deleterious effect on the viability of
the attached algae.
The second layer of the multilayered structure of the present invention is
adjacent to and contiguous with at least a portion of said first surface
of said first layer and is attached to said first surface of said first
layer in such a manner as to substantially maintain said second layer
adjacent to and contiguous with said first surface of said first layer.
The third layer of the multilayered structure of the present invention is
adjacent to and contiguous with at least a portion of said second surface
of said first layer and is attached to said second surface of said first
layer in such a manner as to substantially maintain said third layer
adjacent to and contiguous with said second surface of said first layer.
Each of said second layer and said third layer can be of any material, as
long as at least one of the two layers is at least partially transparent
to actinic radiation. That is, portions of at least one of the layers can
be completely or partially transparent or the entire layer can be
completely or partially transparent.
As a practical matter, especially useful materials for either or both of
said second and third layers are nonwoven webs. If a web is of a
relatively light basis weight, e.g., less than about 34 g per m.sup.2, the
web will be significantly transparent to actinic radiation. Preferably,
the nonwoven webs will be meltblown or spunbonded polyethylene or
polypropylene webs.
FIG. 1A is a perspective view representation of a small section of a
multilayered structure of the present invention in which the first layer
is a continuous sheet formed from a composite of blue-green algae attached
to thermomechanical wood pulp. The second and third layers are meltblown
polypropylene nonwoven webs. FIG. 1B is a similar representation of a
similar structure, except that the first layer is discontinuous composite
consisting of algae attached to thermomechanical wood pulp.
In order to increase the area of the surface facing a light source, the
structure can have a plurality of raised, three-dimensional shapes over at
least a portion of at least one surface. Such shapes preferably will be
bounded by a curved surface. Examples of curved surfaces include, by way
of illustration only, a surface approximating a hemisphere, a zone and
segment of a sphere which is less than a hemisphere, corresponding
portions of an oblate spheroid or a prolate spheroid, combinations
thereof, and other irregular curved surfaces. FIG. 2 is a perspective view
representation of a structure of the present invention having a plurality
of raised, three-dimensional shapes over at least a portion of the surface
thereof, which shapes are bounded by a curved surface approximating a zone
and segment of a sphere which is less than a hemisphere. FIG. 3 is a
cross-sectional view along line 2--2 of one of such shapes. FIG. 3
illustrates a hollow shape pressed into the structure of FIG. 1 while the
sheet is in a moldable state.
Such shapes, however, can be only partially curved or composed solely of a
plurality of planar faces. Examples of these other shapes include surfaces
approximating a cone, pyramid, cube, cylinder, and the like.
The shapes can be on one surface only or on both surfaces. Moreover, the
shapes do not have to be the same and they can be present in a regular
pattern or randomly placed. Preferably, the shapes will be on the surface
which is at least partially transparent to actinic radiation.
A structure having such shapes over a portion of at least one surface is
readily prepared by known means. For example, a structure in a pliant or
moldable state can be passed through at least one pair of pressure rolls,
the first one of which has raised portions over at least a portion of its
surface, with the second roll having depression in its surface to match
the raised portions of the first roll.
In addition, the structure can have one or more holes of regular or
irregular shape through the thickness thereof. For some uses, a single
hole of approximately circular shape is desirable. For example, a
generally circular structure may be sized to fit a pot or container having
a given diameter. A hole in the center of the structure accommodates the
stem or trunk of the plant. Placement of the structure in the pot is
facilitated by slitting the structure from the outside edge to the hole,
preferably along a radius of the structure, as illustrated by FIG. 4A. If
desired, the structure can be cut in half, as shown by FIG. 4B, to
facilitate placement in the pot.
On the other hand, larger structures can have a plurality of holes to
accomodate, for example, a plurality of plants such as might be present in
a greenhouse flat or a garden plot, as illustrated by FIG. 5.
Finally, the structure can have a generally nonplanar configuration in
place of or in addition to such characteristics as those just described.
While agricultural areas, regardless of their size, tend to be essentially
planar, the structure itself can be generally nonplanar. As used herein,
the term "generally nonplanar" means that the thickness of the volume
occupied by the structure is substantially greater that the thickness of
th structure itself. By way of illustra | | |
