a description of the preferred embodiments of the present invention will now be presented with reference to fig1 - 8l . as used herein , the words “ tubes ” or “ tubing ” refer to supply lines for providing water and / or nutrients . as will be appreciated by one of skill in the art , such “ tubes ” or “ tubing ” do not necessarily need to be cylindrical , but may be of any suitable shape , and no limitation is intended by the use of these words . described herein are a system and method of supplying water and / or nutrients to the roots of growing plants wherein the water and / or nutrients are released to the plants as needed by the individual plants . herein the term “ plants ” should be construed broadly , and can include , for example , grasses . although not intended as a limitation on the invention , it is believed that when under water stress , plant roots can emit exudates or surfactants that promote the release of water and / or nutrients stored under the conditions described below . specifically , the plants are supplied water and / or nutrients from supply lines or feeder tubes , at least portions of which are hydrophilic . in some embodiments , the tubing may include a plurality of holes that are covered by hydrophilic membranes ; in other embodiments , the entire tubing , the below - surface portion thereof , or a significant portion thereof is hydrophilic . in yet other embodiments , the system may include a surface tube that is water - impermeable or hydrophobic , the tube being connected to a plurality of hydrophilic tubes that can be inserted into a support medium for supplying the roots . one or more hydrophilic tubes may be inserted into a quantity of support medium such that the tubes are at least partially below the surface of the support media . the support media may be selected from any suitable medium or mixture of media suitable for supporting growing plants and roots . examples , which are not intended as limitations , of such support media can include sand , soil , rockwool , polyurethane foam , fleximat ™, sri cellulose - based growth media , and the like . other suitable media known in the art , such as continuous - fiber growth media , may also be used . in particular embodiments , plants are planted in the support medium and the respective tubes are connected to reservoirs containing water , nutrients , or a mixture thereof . in some embodiments , two tubes may feed a row of plants : a water tube and a nutrient tube . as discussed above , it has previously been shown that the plants are capable of distinguishing between these tubes . alternatively , nutrient ( s ) can be added to a water reservoir for distribution through a unitary tube . thin - walled microporous hydrophilic tubes are not known at present to be commercially available for use as irrigation tubing . in a particular embodiment , hydrophilic materials , including cell - force ™ and flexi - sil ™, may be made into hydrophilic tubes . alternatively , some existing hydrophobic thin - walled tubes can be made hydrophilic by a process that uses a water - insoluble hydrophilic polymer ( e . g ., polyhydroxystyrene , u . s . pat . no . 6 , 045 , 869 , incorporated herein by reference ; structure illustrated in fig6 ) as a surface coating . such solutions applied as a coating to and impregnated with microporous hydrophobic plastic tubing have been shown not to clog the pores and to remain hydrophilic for many years . thus continuous tubes of tyvek ® ( a microporous polyethylene material made from very fine , high - density polyethylene fibers , dupont , richmond , va .) in a radius of 5 - 10 mm ( irrigro - international irrigation systems ) have been used after being made hydrophilic and have been shown to act as a membrane that is responsive to the roots of plants in a subsurface irrigation system . tyvek ® is available in a plurality of styles , each having different properties . although not intended to be limiting , two particular types have been found to be most beneficial for use in the present invention : 1059b and 1073b . as discussed above , it has been shown that hydrophilic membranes can become hydrophobic over time owing to organic impurities in the water adsorbed onto the membrane . because of the variability of the impurities in water , we have added organic substances to the water which can be adsorbed onto the exit pore walls , making the membrane hydrophobic , and thereby reducing the flow of water or nutrient solution through the membrane . examples of suitable organic substances include , but are by no means limited to , humic acid , kerosene , turpentine , pinene , paraffin , and hexadecane . in other embodiments , other suitable c8 - c16 saturated hydrocarbons may be used . the amounts added ranged from 10 ppb to 10 ppm to the irrigating medium . as will be appreciated by one of skill in the art , in some embodiments , the addition of the organic substance may not be essential , depending on the quality of the water . when growing crops in soil , the addition of nutrient on a continuous basis is not essential ; however , when growing crops in sand , fleximat , or rockwool , a nutrient solution , for example , any suitable nutrient solution known in the art such as those commonly used in hydroponic systems , e . g ., hoegland solution , peter &# 39 ; s solution , miracle - gro , or other less dyed fertilizer such as schultz export may be added to the water supply or may be fed directly to the plants in a separate tube , as described above , and thus the roots of the plant can be allowed to take as much water and nutrient as required . however , for growth in artificial media the inclusion of nutrients and micronutrients is important . fig1 a and 1b illustrate a system 10 that uses twin irrigation tubes 11 , 12 for delivering water and nutrient solution to plants 13 growing in a growing medium 14 . in this embodiment 10 , the tubes 11 , 12 are running through the root systems 15 of the plants 13 . it has been found in experiments in both sand and potting soil that the higher the concentration of nutrients used , the smaller the volume of the nutrient solution that is released to the roots 15 , which is illustrative of the water conservation achieved by the current invention . it will be understood by one of skill in the art that the tubes 11 , 12 could be provided as a single composite double - lumen tube without departing from the spirit of the invention . the diameters of the two portions could be in a proportion commensurate with a plant &# 39 ; s requirements for water versus nutrient , for example , double the size for the water tube , although this is not intended as a limitation . in some embodiments , since subsurface thin - walled microporous tubing can be collapsed if sufficient pressure is applied , a spiral 60 comprising , for example , plastic , can be incorporated into a tubing such as tubing 11 or 12 to form a tube 61 that is more resistant to collapsing ( fig1 c ). fig2 illustrates a system 20 for the irrigation of grass 21 where the subsurface tubes 22 are spaced 1 - 2 feet apart and are substantially continuously fed with water under low constant pressure , with nutrients added to the aqueous solution as desired . the irrigation systems and methods described herein are believed superior to any other watering system currently in use , and further are independent of atmospheric pressure , making them usable for astroculture or micro - gravity conditions , as well as others . in one embodiment of the invention 30 ( fig3 ), for example , a continuous fiber growth medium 31 such as rockwool or the spongy fleximat ( from grow - tech ) can be used to support the plants 32 and their roots 33 . in this embodiment 30 , both of the reservoirs 34 comprise a container 35 that has an interior space 36 for holding the water and nutrient solution therein . the containers 35 are formed similar to a bellows , and are movable between an expanded state when containing solution and a retracted state when solution has been removed . the containers 35 also comprise a filling inlet 37 that is in fluid communication with the containers &# 39 ; interior space 36 for adding solution thereto . distribution tubes 38 are also in fluid communication with the containers &# 39 ; interior spaces 36 and with inlets 39 of the hydrophilic tubes 40 . this arrangement provides solution to the tubings &# 39 ; lumina 40 . the distribution tubes 38 also have check valves 41 therein for preventing backflow of solution from the tubes 40 toward the containers &# 39 ; interior spaces 36 . support for plants and their roots can be provided for in the present system under zero gravity , for example , with the use of a monolithic contiguous material such as rockwool or fleximat , a spongy hydrophilic porous material made by grow - tech or the newly developed artificial sponge such as , for example , agri - lite ( sri enviro - grow ). by using these materials to surround twin microporous hydrophilic irrigating tubes , one supplying water while the other supplying a nutrient solution , it is possible to achieve complete conservation of water and nutrients supplied to growing plants . such a system can also be applied to arid or desert environments where water conservation is desirable . early laboratory tests showed that using nutrients in water , it was possible to grow tomatoes in sand with amerace a10 membranes 42 ( 50 % silica gel in polyethylene ) glued over holes 43 in a subsurface pvc tube 44 ( fig4 ). the holes 43 in the pvc tube 44 were 12 mm in diameter , spaced 10 cm apart , drilled in 17 - mm - id rigid pvc tubing . the holes 43 are believed to have limited the amount of water and nutrient available to the growing plant , and the system proved to be inadequate when the plants began to bear fruit and needed more membrane area to supply the plants &# 39 ; requirements . increasing the total surface area of the membrane by drilling and covering more holes improved the system . however , a best mode of practicing the invention at the present time favors the use of a continuous tube . because of the brittle nature of amerace , membrane tubes made of this material tended to crack and leak . tyvek ® ( dupont ) in tube form has been used for irrigation purposes under elevated water pressure for gardens and row crops . however , the hydrophobic nature of the polyethylene material permits it to act as a drip source of water for plants without any control by the exudates of the plant roots . the conversion of a hydrophobic surface to hydrophilic has been described ( u . s . pat . no . 6 , 045 , 869 ) and can be used to make tyvek ® tubing hydrophilic and responsive to the water and / or nutrient needs of the plant . when the tubing has been made hydrophilic by coating and impregnating it with an alcohol solution of polyhydroxystyrene , the tubing was found to be permeable to water at much lower pressures , and showed a decrease in water permeability as the organic compounds in water are adsorbed onto the exit pore walls . this can be considered a “ conditioning phase ,” during which permeability can be decreased by as much as 80 % by the addition of hydrocarbons to the tap water . the present invention is believed to be the first to provide a plurality of feeding tubes arranged to extend beneath the surface of a support medium to feed a plurality of plants or a row of plants . furthermore , a clear advantage of tubes comprising a hydrophilic material is that a greater area of the support medium is fed water and nutrients compared to a single horizontal membrane . the invention will now be described by way of examples ; however , the invention is not intended to be limited by these examples . a 4 ft . length of tyvek ® tubing (# 1053d ) was made hydrophilic with an alcoholic solution of polyhydroxystyrene and submerged in a 4 . 5 ft by 13 cm wide by 10 cm deep planter , covered with soil and connected to a constant supply of nutrient solution at a constant head of 35 cm of water . ten cherry tomato ( lycopersicon sp .) seedlings were planted at even distances next to the tube where water and nutrients were supplied . fluorescent lighting was supplied to the plants for 18 hours per day . the average consumption of water was 75 ± 10 ml / hr when the plants were 15 cm high and 125 ± 20 ml / hr when the plants were 25 cm high . when rainfall was simulated by spraying the bed with 100 ml of water , the consumption of water dropped to zero for 2 hours and slowly over the next 3 hours returned to the normal rate . the plants grew to two feet in height , and numerous tomatoes were harvested . at the end of the experiment , the system was examined to determine if there was any competition between the plants for space on the membrane . an examination of the root system indicated that the roots encircled the membrane only within about 1 - 2 inches from the plant stem . this indicates that it should be possible to increase the density of plant growth to an extent that would only be limited by the photochemical flux available and mutual interference . when a dual - tube system was used to supply both water and nutrient separately , the ratio of water consumed to nutrient solution consumed was approximately 2 . 5 to 1 for 8 cherry tomato plants in sand . again , little or no fluctuations were observed when the size of the plants reached a height of 35 cm . a continuous irrigation tube can be unnecessary for plants such as grape vines or kiwi vines that are spread apart from each other by distances as much as 20 to 40 cm . in these situations 50 , it is more practical to use a main flexible surface distributing tube 51 of from 20 - 30 mm id , out of which are drawn satellite tubes 52 that feed a short length of from 10 to 30 cm , depending of the size of the vine , of thin - walled microporous hydrophilic irrigating tube 53 , closed at its end 54 , surrounding the roots 55 of the vine or bush 56 , as illustrated in fig5 a and 5b . a tomato plant was planted in potting soil , into which was also placed two 20 - cm - long microporous hydrophilic tubes of 1 cm radius . the tubes were connected to reservoirs of water and nutrient which were kept full . the soil remained dry while the plant grew to produce numerous tomatoes . another experiment was conducted with tyvex ® tubing (# 1053b ), 1 . 25 m long and 1 cm radius . the tubing was sealed at one end that was made hydrophilic with a 3 % solution of polyhydroxystyrene ( novolac grade from triquest ) in ethanol . the tubing was submerged in a 1 . 4 - m planter , covered with soil , and connected to a supply of nutrient solution at a constant head of 35 cm of water . ten cherry tomato ( lycopersicon sp ) seedlings were planted at even distanced next to the tube , by which water and nutrients were supplied . the plants grew during the conditioning phase while exposed to fluorescence lighting for 16 hr / day . the average consumption of water was 75 ± 10 ml / hr when the plants were 15 cm in height and 125 ± 20 ml / hr when the plants were 25 cm in height . rainfall was simulated by spraying the bed with 100 ml water , following which the consumption of water dropped to zero for 2 hours and then slowly , over the next 3 hours , returned to the normal rate . the plants grew to 60 cm in height , and an abundance of tomatoes was harvested . at the completion of the experiment , the system was examined to determine if there had been any competition between the plants for space on the membrane . an examination of the root system indicated that the roots encircled the membrane only within about 2 . 5 - 5 cm from the plant stem . this finding would seem to indicate that it should be possible to increase the density of plant growth to a level only limited by the light flux available and mutual interference . it has also been shown that different plants requiring different rates of water and nutrient can grow together with each being satisfied individually without monitoring . when a dual membrane system was used to supply both water and nutrient separately , the ratio of water consumed to nutrient solution consumed was approximately 2 . 5 to 1 for 8 cherry tomato plants in sand . once again , there was little or no fluctuation observed when the size of the tomato plants reached a height of 35 cm . a planter 115 cm long , 13 cm wide , and 10 cm deep , was set up in a greenhouse with dual - feed membrane tubes for water and nutrient through the center of a bed comprising 50 cm of flexmat and 50 cm of rockwool separated by 15 cm of polyurethane foam . the seeds or seedlings of canola ( brassica sp ), beans ( phaseolus sp ), corn ( zea mays sp ), and tomatoes ( lycopersicon sp ) were planted in each of their respective media and their growth patterns observed . growth , which was favored in the fleximat , proceeded normally , except for the polyurethane foam , with each crop growing at its own rate under a light flux of 50 - 60 mw / cm 2 . root crops such as carrots ( daucus carota var sativa sp ), radishes ( raphanus sativus sp ), beets ( beta vulgaris sp ), and onions ( allium sp ) were grown in soil and peat , while potatoes ( solanum tuberosum sp ), parsnips ( pastinaca sativa sp ), and parsley ( petroselinum sativum var tuberosum sp ) were grown successfully in vermiculite . a cellulose material ( sri petrochemical co .) can also be used as an artificial growth medium . it was determined that grass ( gramineae sp ) can be successfully irrigated for 3 successive years with submerged tubular membranes spaced 40 - 50 cm apart . in another case , two hydroponic planters ( 30 × 30 × 30 cm ) were fitted with a membrane tube for a water / nutrient solution approximately 7 cm from the bottom . the media comprised a soil - less mixture approximately 25 - 26 cm deep in the planters . this depth allowed the root crops to produce straight tap roots , which is of concern to consumers when purchasing vegetables . one planter was seeded with parsnips ( daucus carota var . sativa sp ). the other planter was seeded with parsley ( petroselinum sativum var . tuberosum var . tuberosum sp ), a dual - purpose crop of foliage and root stocks . plant competition controlled the over - seeding issue with each planter . the plants received only natural sunlight , reducing the risk of “ bolting .” extreme warm temperatures were a concern for the health of the plants . the parsnip roots were straight in growth , and produced a total weight of 38 . 9 g . the texture and flavor were excellent . the parsley produced straight tap roots , giving a total weight of 38 . 3 g . the foliage produced had longer petioles than usually purchased , yet the total weight was 58 . 9 g . it will be appreciated by one of skill in the art that plants with varying water requirements can be satisfied by the embodiments of the present invention , wherein one continuous porous hydrophilic irrigating tube is used to allow each plant to take its water requirements independently of the other plants . such requirements are often needed in greenhouses , where many different plants are cultivated under one roof . it has also been shown that a hydrophilic irrigation tube with two channels , one for water and the other for nutrients , can fully satisfy the plants &# 39 ; requirements and also increase the density of the plants , limited only by the sunlight available . it has also been shown that commercially available thin - walled microporous hydrophobic tubes can be converted to hydrophilic tubes and thereby become responsive to plants and their roots . such tubes may include , but are not intended to be limited to , high - pressure irrigation hoses , although their use in the present invention does not require the use of high pressure . it has also been shown how a dual - membrane tube can be incorporated into a container for one or more plants so that the plants can be fed on demand both water and nutrients from separate reservoirs and thereby require no attention or supervision as long as there is water available in the tube reservoirs . in a particular embodiment , a diametric ratio of 3 : 1 for the water tube over the nutrient tube is optimal , although this is not intended as a limitation , and obviously is dependent upon nutrient concentration and plant type . it has additionally been shown that water systems that are free of contaminated organic substances and unresponsive in the irrigation system can , by the addition of trace amounts of one or more hydrocarbons to the water supply , become responsive to the irrigation system . it has also been shown that the irrigation system of the present invention can be used to replace the emitter in a drip irrigation system , thereby making the release of water and / or nutrient responsive to the roots . in a particular embodiment , a factor of from 100 to 500 has been found for the difference in water volume used between the known drip irrigation systems and that of the present invention . sectors of grass are known to be grown substantially in isolation , for example , on golf courses wherein the greens are formed within soil - filled depressions in the ground and continuously or at predetermined intervals fed with water and nutrients . in such an arrangement , the system of the present invention can ideally provide water and nutrients to the grass roots on an on - demand basis , thereby saving both water and nutrients , and also ensuring optimal sustenance of the greens . the following tables 1 - 4 include data on experiments conducted indoors ( table 1 ) and outdoors ( table 2 ), and the flow rates for water and nutrient ( table 3 ) and for watering results in series and for single plants ( table 4 ). a planter with two tubes , one for water ( w ), the other for nutrient solution ( n ). the reservoirs were interchanged periodically to cancel any membrane effects . flow rates in ml / hr ; experiment time march 18 to july 16 . another aspect of the invention is directed to the making of tubing for use with a “ water - on - demand ” system . in one method , sheets of a low - porosity substance are coated with the aforementioned polyhydroxystyrene , and formed into cylinders by , for example , thermal , ultrasonic , or impulse means . although not intended as a limitation , a possible explanation of the operation of the polyhydroxystyrene polymer ( fig6 ) will now be presented . first , how the polyhdroxystyrene attaches to the membrane : polyhydroxystyrene has two groups , an hydroxyl ( oh ), which is hydrophilic and can hydrogen bond with water , and the styrene groups , which include a benzene ring (— c 6 h 4 —) attached to an ethylene group (═ ch — ch 2 —), both of which are hydrophobic and can stick to the hydrophobic polyethylene membrane , leaving the hydrophilic ( oh ) group , which forms a weak hydrogen bond with water . as discussed above , the polymer can act as a capillary through the membrane . it has been shown that organic impurities in water are 10 5 - 10 6 times more likely to stick at the exit end wall of the capillaries , where there is a gas - liquid - solid equilibrium ( i . e ., air - water - membrane ). the organic impurities are in equilibrium along the walls of the capillary , where the equilibrium is only between liquid and solid . thus the surface of the exit pores become hydrophobic due to the adsorption of the trace organic impurities in water and / or nutrient solution . when a plant is in need of water , it emits chemicals called exudates that can include a surfactant that removes the adhering organic compounds at the exit wall and liquid from the irrigation tube now is allowed to flow . this has been shown for two different membranes in the prior art , as discussed above with reference to fig7 - 8l . high - purity water is free of organic impurities . some domestic water supplies are often purified to such an extent that very little organic impurities remain . this would result in pore closure only after a large , and usually unnecessary , volume of water had passed through the membrane . the result would not be suitable because of the time delay between the removal of the organics and their deposition onto the membrane and the closure of the pores . on the other hand , too much organic content in the water could result in a delay in opening the closed pores because of the limited amount of surfactant that is released by the roots . it has been found that in general the membrane area needed for a plant is best supplied by a tube of diameter equal to about a 1 - cm radius , with a thickness of 0 . 5 mm maximum and pore sizes of from 0 . 1 to 5 μm , with a preferred average of 0 . 4 μm , although this is not intended as a limitation , and other porosity values can be used . this segment of the membrane is to be in contact with the roots of the plant . short segments of membrane tubing can be supplied with water and / or nutrient solution by smaller diameter tubing , but care must be taken to prevent air locks in the line . tubing of 1 - cm id would not be considered too large . since the feed lines are exposed to light ( sunlight or artificial lighting ), it is necessary to use opaque tubing , or the solar active light will result in algae formation that can eventually block the pores . it is believed that the coating of the hydrophobic membrane is primarily to allow the resulting hydrophilic surface to become hydrophobic and to close the pores . leaving the inner pore uncoated would restrict the flow of water through the membrane . in the foregoing description , certain terms have been used for brevity , clarity , and understanding , but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art , because such words are used for description purposes herein and are intended to be broadly construed . moreover , the embodiments of the apparatus illustrated and described herein are by way of example , and the scope of the invention is not limited to the exact details of construction . having now described the invention , the construction , the operation and use of preferred embodiments thereof , and the advantageous new and useful results obtained thereby , the new and useful constructions , and reasonable mechanical equivalents thereof obvious to those skilled in the art , are set forth in the appended claims .
a method for efficiently delivering an aqueous solution to plants includes positioning a distal portion of a microporous hydrophobic tubing coated with a hydrophilic polymer adjacent a root system of a plant and channeling an aqueous solution from an inlet to the distal portion through a lumen . the tubing along the distal portion has a porosity adapted for permitting a flow of the aqueous solution therethrough when acted upon by a surfactant root exudate generated by the roots due to water stress . the aqueous solution is held in a reservoir , from which it can be channeled to the hydrophilic device &# 39 ; s inlet . a nutrient solution can also be channeled to the plant roots via additional tubing .