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Analyses of 21 aquatic plants are tabulated and compared with alfalfa (lucerne) hay. All the analyses are on a dry matter basis.

Proximate Analyses of Aquatic Plants and Alfalfa Hay

PlantCrude protein %Ash %Ether extract %Crude fibre %Nitrogen-free extract %
Alfalfa hay,16.918.842.1031.6040.55
Anacharis canadenis14.704.491.0711.3968.35
Carex lacustris8.63.962.9627.4160.04
Carex stricta9.94.90.8929.4158.86
Ceratophyllum demersum17.002.181.5115.2064.11
Chara vulgaris7.925.62.127.6577.56
Eleocharis smalli5.781.641.0927.0364.46
Lemna minor17.861.612.1911.8266.52
Myriophyllum exalbescens12.281.67.2113.4972.35
Nuphar variegatum15.70.962.4623.1357.75
Potamogeton amplifolius14.362.391.5315.7266.00
Potamogeton pectinatus14.053.22.0915.6467.00
Potamogeton richardsonii11.202.621.1319.1165.94
Sagittaria cuneata21.811.911.3717.3457.57
Sagittaria rigida14.782.271.7923.6957.47
Sparganium eurycarpum7.602.47.7120.5768.65
Sparganium fluctuans13.191.041.6714.4269.68
Typha angustifolia6.92.93.9827.4963.68
Vallisneria americana15.153.10.9727.3253.46
Zizania aquatica9.882.441.1027.4359.15

Mineral Contents of Aquatic Plants and Alfalfa Hay

Alfalfa hay1.
Anacharis canadensis5.
Calla palustris1.02.603.19.128.298123149823.8
Carex lacustris.
Carex stricta.
Ceratophyllum demersum2.
Eleocharis smalli1.
Lemna minor2.
Myriophyllum exalbescens.
Nuphar variegatum.
Nymphaea tuberosa2.
Potamogeton amplifolius1.01.832.91.172.232165150541.0
Potamogeton pectinatus3.76.341.99.107.200117153521.0
Potamogeton richardsonii4.
Sagittaria cuneata.78.552.82.391.354190416128.2
Sagittaria rigida.99.311.82.243.432208323630.2
Sparganium eurycarpum.
Sparganium fluctuans.
Typha angustifolia.
Vallisneria americana1.
Zizania aquatica.

The authors comment that the ash values for the aquatic plants (averaging 2.1%) were lower than for alfalfa hay. The figures given relate only to the minerals within the plants and do not include any surface contamination by soil or crustaceans. Normally submerged plants are found tobe higher in ash content than emergent plants because of such contaminations. They may also be inherently richer in minerals but generally lower in ether extractable material.

Crude fibre, averaging about 19.5%, was found to be lower for all aquatic plants than alfalfa hay, due probably to the lower need for structural strength by plants supported by water buoyancy.

Average contents of Ca, 1.6%, P, 0.27%, K and Mg were all similar to those of land forages, though as shown in the tables there was considerable variation between species.

Detailed analyses of neutral detergent fibre, acid detergent fibre content, acid detergent lignin, hemicellulose and cellulose content of all the 21 plants tabled above were again compared with alfalfa hay.

Linn, J.G. et al., 1975a Nutritive value of dried or ensiled aquatic plants. 2. Digestibility by sheep. J.Anim.Sci., 41(1):610–5

A set of analyses comparing Myriophyllum exalbescens with Potamogeton pectinatus and with alfalfa (lucerne) hay is given in this paper.

*Little, E.C.S. and I.E. Henson, 1967 The water content of some important tropical water weeds. PANS (C), 13(3):223–7

Analyses of water content of plants grown in the United Kingdom from specimens collected from different countries:

PlantOriginWater (% Fresh weight)
Whole plantLeaf bladePetioleStemRoots
Water hyacinthSudan (Nile)91.788.193.992.486.7
 Congo River91.687.193.393.088.7
Pistia stratiotesGuyana92.394.0-92.488.2
Salvinia auriculata (= molesta)Lake Kariba92.892.5-92.693.3

The water content of various fodder crops (%) is cited from Woodham, H.E. (1954):

Pasture grassesKaleMaizeLucerneTurnips

The authors state that their results suggest that a typical water content of floating aquatic plants is about 92%. This compares with 92% determined for a submerged water weed, Elodea, by Fish and Will (1966). Thus to obtain the same dry matter of plant material from water weeds about twice as much fresh material is needed as for lucerne, or 2.5 times that for pasture grasses.

*Loosli, J.K., et al., 1954 The digestibility of water hyacinth (Eichhornia crassipes) by silage by sheep. Philipp.Agric., 38(2–3):146–8

Water hyacinth (fresh)compared with water hyacinth silage (%):

Silage (mean of three)

Maciejewska-Potapczykowa, 1970 W., L. Konopska and E. Narzymska, Proteins in duckweed (Lemna minor). Acta Soc.Bot.Pol., 39(2):251–5

Analysis of Lemna minor showed that 16% of dry matter was protein. The amino acid composition was similar to that of hen egg protein.

Mackenthun, K.M., 1962 A review of algae, lakeweeds and nutrients. J.Water Pollut.Control Fed., 34:1077–85

The author reviews the literature of yield and nutrient contents of aquatic plants and plankton in the U.S.A. He cites Lake Mendota, Wisconsin, where the 0 to 1-m zone contained 1 800 kg/ha of submerged weeds on a dry weight basis, the 1- to 3-m zone 2 700 kg/ha, and the 3- to 7-m zone 1 450 kg/ha. On Lake Green, Wisconsin, the corresponding figures were 660 kg/ha, 2 200 kg/ha and 1 775 kg/ha. Analyses gave 12% dry matter, 1.8% N and 0.18% P. A typical nutrient content of all the plants in a lake would be: N, 36 kg/ha, and P, 3.6 kg/ha.

Plankton content of Lake Mendota was found to vary seasonally ranging from:

spring - 400 kg/hasummer - 140 kg/ha
autumn - 360 kg/hawinter - 110 kg/ha (dry weight).

A typical population could thus tie up about 36 kg/ha N and 3.6 kg/ha P.

Blue-green algae are 6.8% N (approx.) and 0.69% P. Thus an algal population could theoretically tie up about 17 kg/ha N and 1.7 kg/ha P. This is about half the quantity held by a normal submerged weed population.

Mackenthun, K.M., Nutrients and their relationship to weed and algal growths. 1971 Hyacinth Control J., 9(1):58–61

The author has searched the literature and from it abstracted the following figures:

Crop per Lake Weed Hectare
 PhytoplanktonAttached algaeSubmerged vascular plants
Wet weight (kg)1 120–4 0002 24015 700
Dry weight (kg)112 – 4002242 000
% N (dry weight)
% P (dry weight)0.690.140.18
N in crop (kg)7.9 – 286.836
P in crop (kg)0.8 –

Moore, A.W., 1969 Azolla: biology and agronomic significance. Bot.Rev., 35(1):17–34

Reviewing the literature on Azolla spp. the author cites analyses from Japan as:

crude protein, 23.8%; crude fats, 4.4%; crude fibre, 9.5%; strach, 6.4%.

In a table of analyses from nine sources, including Australia, Japan, Indonesia and North Vietnam, the following is the range of the results given from A. pinnata and A. japonica:

 %Fresh weight% Dry matter
 92.8 – 93.6 3.37 – 4.75 0.12 – 0.95 0.88 – 6.52 0.30 – 2.49 0.25 – 0.41

Yields of 50 t fresh weight/ha/year have been reported from Vietnam. From the U.S.A. a yield of 7 t of fresh Azolla in 3–4 months from 0.1 ha was reported. Assuming a moisture content of 93.3%, and with a dry matter content of 4% N, the author calculates that this represents a yield equivalent to 160 kg N/ha. From India a yield of Azolla on ponds from November to January of about 2 200 kg dry matter/ha was recorded, containing about 90 kg N/ha. In small plot experiments in Japan, having added superphosphate, a yield of 140 kg N/ha was recorded in a period of seven weeks.

Discussing these figures in relation to the known nitrogen-fixing capacity of Azolla (see Chapter IX), the author comments: “It is evident that Azolla can assimilate in 3–4 months 100–160 kg N/ha. It is likely that at least half of this N is derived from the atmosphere.”

Mukhtar, A.M.S., A preliminary study on the chemical composition of water hyacinth 1967 (Eichhornia crassipes) in the Sudan. Sudan J.Vet.Sci.Anim.Husband., 8(2):

Analysis of water hyacinth on air-dry basis (%):

 Dry matterCrude proteinEther extractCrude
Whole plant93.45.70.622.920.064.2

The author points out the similarity of analyses of the different parts of the plant except for the higher ash content of the roots.

Average yield was 310 tonnes/ha.

Nelson, W.J. and L.S. Palmer, 1938 Nutritive value and chemical composition of certain fresh-water plants of Minnesota. Tech.Bull.Min.Agric.Exp.Stn., (136):1–34

The authors review the literature on analyses of Elodea, Myriophyllum, Vallisneria and other species since 1904 and point out the variability of analyses recorded. They indicate the large quantities of potential forage represented by the water weeds in the lakes of Minnesota, which cover 5 600 sq. miles (1.45 million ha) with an estimated annual yield of over 860 000 t of dry matter, containing at least 124 000 t of protein; this yeild is independent of droughts which may be experienced by dry-land farming nearby.

A summary is given below of some of their analyses of three species compared with earlier work on the same species in a similar locality. This illustrates the variations that may be observed. The figures are also compared with analyses made for a common fodder crop, alfalfa (lucerne).

  %Dry weight 
 ElodeaMyriophyllum sp.Vallisneria sp.Alfalfa
Crude protein26.825.818.815.211.817.3
Crude fibre15.414.115.015.814.035.6
Dry matter % fresh7.513.

1 W.J. Nelson and L.S. Palmer, 1938

2 H.A. Scheutte and A.E. Hoffman, 1921

3 H.A. Scheutte and H. Alder, 19274

4 H.E. Woodman et al., 1933

Parra, J.V. and C.C. Hortenstine, 1974 Plant nutritional content of some Florida water hyacinths and response by pearl millet to incorporation of water hyacinths in three soil types. Hyacinth Control J., 12:85–90

The means of analyses of water hyacinth from 19 different locations are given below:

 % Dry weightStandard deviation
C/N ratio23.37.0

 ppm Dry weightStandard deviation
Al2 5682 668
Fe2 7723 765
Cr0 to 35-
Pb0 to 20-

The authors point out the high Al content which could be hazardous if the hyacinths were used to fertilize aluminium-sensitiye crops. The high Fe content would be beneficial.

Parra, J.V. and C.C. Hortenstine, Response by pearl millet to soil incorporation of water hyacinths. J.Aquat.Plant Manage., 14:75–9

Water hyacinth analyses:

% Dry weightppm
N1.2Fe1 002
K3.6Al1 244

Pirie, N.W., 1970 Weeds are not all bad. (Water hyacinths and other pests can also be good animal fodder). Ceres, 3(4):31–4

The nitrogen content of a range of aquatic plants is tabulated:

Popular nameScientific nameN% dry matter
Alligator weedAlternanthera philoxeroides1.3–3.5
CoontailCeratophyllum demersum2.7–3.0
Water hyacinthEichhornia crassipes1.3–3.7
Water hyacinth
(leaves only)E. crassipes5.0
Canadian pondweedElodea canadensis2.2–6.3
Water bind-weedIpomoea reptans4.6–5.8
Water willowJusticia americana1.6–3.8
Duck weedLemna minor2.5–5.0
MilfoilMyriophyllum spicatum1.8–4.1
ReedPharagmites communis1.8
Water lettucePistia stratiotes1.7–3.9
PondweedPotamogeton sp.1.1–3.5
SalviniaSalvinia auriculata0.8–1.8
Reed-maceTypha sp.0.5–2.4

The author estimates the annual yield of aquatic weeds at 5–10 t/ha.

Pirie, N.W. (Ed.), 1971 Leaf protein: its agronomy, preparation, quality and use. IBP Handb., (2):192 p.

Chapter 3 of the above Handbook, “A survey of other experiments on protein production”, was written by the editor himself. Referring to weeds as a source of protein, he comments that aquatic weeds should be considered separately as a potential crop because each area of water tends to be dominated by a single species and the water often contains nutrients such as N, P and K sufficient to encourage luxuriant growth. Many analyses have been published which are often not accompanied by an adequate description of the physiological state of the material analysed. He cites the table, shown in the reference on p. 36, of representative analyses taken from many sources. He says that it shows the wide variation in N content that has been found for the same species. It also shows that, when harvested at suitable times, some water weeds contain more N than conventional crop plants.

He cites (p. 37) the following potential yield figures (kg/ha):

 Standing cropAnnual
Eichhornia crassipes50 000120 000
Pistia stratiotes20 00070 000
Ipomoea reptans25 00080 000

He adds (p. 38) figures for the N and protein content of the same species.

SpeciesStage of growth% DM on pulp% N on DM of pulp% N on DM of fibrepH of extractTotal N as % of N in pulpProtein N as % of N in pulp% N on dry crude protein
E. crassipesFlowering8.03.33.2-1267.6
P. stratiotesPre-flowering6.
I. reptansPre-flowering9.93.352.95.432259.8

Quimby, P.C. and E.L. Robinson, 1975 Calcium oxalate in alligator weed and silverhead. Proc. Annu.Meet.South.Weed Sci.Soc., 28:264–5 (Abstr.)

Calcium oxalate (insoluble) analyses (%dry weight) in different parts of two related plants:

Alternanthera philoxeroides (alligator weed)1.960.02
Philoxerus vermicularis (silverhead)3.480.07

Terrestrial A. philoxeroides plants, fertilized with N at 448 kg/ha, contained 4.40% water soluble oxalate and 5.10% insoluble oxalate, compared with unfertilized plants which had 2.8% and 1.6%, respectively.

In comparison, nodes of A. philoxeroides from a eutrophic lake contained only 0.6% and 1.90%of soluble and insoluble oxalates, respectively.

Rao, K.V., 1973 A.K. Khandekar and D. Vaidyanadham, Uptake of fluoride by water hyacinth, Eichhornia crassipes. Indian J.Exp.Biol., 11(1):68–9

Analyses of different parts of water hyacinth (ppm dry weight):

ElementLaminaPetioleBase of petioleRoot
Ca23 20022 20026 20012 100
Mg6 00012 60019 2001 800
Na179622 10046
K2 8309 99932 5001 665
Fe6522321 2576 000
Ti1863725601 332
P121291 039117

Ash on dry weight basis of whole plant = 13.5%.

Reay, P.F., The accumulation of arsenic from arsenic-rich natural waters by aquatic 1972 weeds. J.Appl.Ecol., 9(2):557–65

Analysis of arsenic content in various plants growing in arsenic-enriched water (geothermal) and low-arsenic (non-geothermal) water in New Zealand:

SpeciesArsenic content in geothermal water(mg/kg dry weight) in non-geothermal water
Ceratophyllum demersum6501.4
Lagarosiphon major251-
Elodea canadensis3073.0
Potamogeton sp.178<6
Lemna sp.302.5
Nitella hookeri18213.0
Azolla rubra-<2
Compsopogon hookeri550-
Enteromorpha nana20-
Typha orientalis8-
Scirpus sp.12-

Reay points out that the arsenic concentrations in the various plants were influenced by the concentration of this element in the water and not by the amounts in river and lake bottoms. The Waikato river, receiving arsenic from geothermal discharges, contained 30–70 μg/litre. Marine plants may contain up to 60 mg As/kg (dry weight), though most contain 10–20 mg/kg. This reflects the lower arsenic content in the sea of 2–5 μg/litre. Reay draws attention to the relatively low arsenic content in the emergent aquatics T. orientalis and Scirpus sp. (monocotyledons) and the high concentration in a red alga, C.hookeri, compared with a green alga, E. nana.

Riemer, D.N. and S.J. Toth, 1969 A survey of the chemical composition of Potamogeton and Myriophyllum and in New Jersey. Weed Sci., 17(2):219–23

The authors carried out detailed chemical analyses of several species of Potamogeton,Myriophyllum and other genera in the family Nymphaeaceae. Wide vari observed both between species and within species. A table of means was given:

ElementPotamogeton spp.1Myriophyllum spp.2
 % dry weight% dry weight

1 including P.amplifolius,P.crispus,P.natans,P.pulcher and P.pectinatus.

2 including M.heterophyllum and M.spicatum.

Analyses were made of a range of plants growing within the same body of water, which confirmed the wide difference in uptake that can occur between different species. For example, Na ranged from 0.20% in P.pulcherto 2.12% in Cabomba caroliniana. The N content of P. pulcher was 1.87% while it was 3.30% of M. heterophyllum.

Riemer, D.N. and S.J. Toth, Chemical composition of five species of Nymphaeaceae. 1970 Weed Sci., 18(1):4–6

Chemical analyses were made of different parts (leaf blades, petioles, top growth) of Nuphar advena, Nymphaea odorata and N. tuberosa, Brasenia schreberi and Cabombaue/caroliniana. Mean analyses:

ElementN. advenaN. odorataN. tuberosaB. schreberiC. caroliniana
   % dry weight  

As in the analyses carried out by Riemer and Toth and tabulated in the reference above, variations within the different parts of the plant and between species ranged from low to high.

An interesting point of detail was the narrow range of Fe concentration, for N.advena, in leaf blade and petiole. This suggested to the authors that this species may not have the Fe precipitates on its surfaces which have been reported for some submerged species.

Riemer, D.N. and S.J. Toth, 1971 Nitrification of aquatic weed tissues in soil. Hyacinth Control J., 9(1):34–6

Nitrogen analyses of a range of aquatic plants compared with a fodder crop:

Scientific nameCommon name% N (dry wt.)
Myriophyllum heterophyllumWater milfoil, broadleaf3.51
Cabomba canadensisCabomba2.52
PeltandravirginicaArrow arum3.63
NupharadvenaSpatterdock (leaves)5.65
N.advenaSpatterdock (petioles)2.43
Utricularia sp.Bladderwort3.57
Sparganium sp.Burreed2.54
PotamogetonpulcherHeartleaf pondweed2.11
PhragmitescommunisCommon reed1.97
MedicagosavitaLucerne (alfalfa)3.77

Siriwardene, J.A. de, 1970 S.S.E. Ranawana and G.A. Piyasena, study of the feeding value of Salviniaauriculata for growing pigs. Trop.Agric., Colombo, 126(1):31–4

Analysis of Salvinia auriculata (= molesta) compared with a grass, Brachiaria

   % Wet weight   
 Dry matterCrude proteinEther extractCrude fibreAshNFE
S. auriculata22.
B. brizantha4.

* Smetana, P., 1968 Water hyacinth compared with elephant grass. Extract from report to the Government of Burma, by P. Smetana, FAP Poultry Expert, Burma. In Handbook of utilization of aquatic plants, edited by E.C.S. Little. Rome, FAP, Plant Production and Protection Division, PL:CP/20:51

Analysis (% fresh weight) of water hyacinth, Eichornia crassipes, compared with elephant grass, Pennisetum purpureum:

 WaterCrude proteinFibreCarotene (IU/100 g)
Water hyacinth882.042.4213 100
Elephant grass843.074.1316 705

These figures were later rearranged and converted to composition in grams/100 g dry matter, by W.W. Thomann, Poultry Specialist, FAO:

 Crude proteinFibreCarotene IU
Water hyacinth17.020.2109 000
Elephant grass19.225.8104 000

Steward, K.K., 1970 Nutrient removal potentials of various aquatic plants. Hyacinth Control J., 8(2): 34–5

Some of the literature on analysis, including yields, of different types of aquatic plants is reviewed.

SpeciesSourceAnalysis % DMAnnual yield DM
Submerged plants    
Thalassia testudinumPuerto Rico4.00.433.5
HydrillaverticillataU.S.A. (Florida)
Myriophyllum exalbescensU.S.A. (Wisconsin)
Emergent plants    
Typha latifoliaU.S.A.(Minnesota)1.50.1851.5
Floating plants    

Steward points out the lower yields and hence nutrient content of submerged weeds, attributed to the less abundant supply of carbon dioxide and light to which they have access, compared to emergent or floating plants.

Sutton, D.L. and W.H. Ornes, Phosphorus removal from static sewage effluent using duck– 1975 weed. J.Environ.Qual., 4(3):367–70

In the course of the experiments reported, analyses (% dry weight) of a mixed population of Lemna gibba and L. minor gave the following results:

crude protein = 10.6; P = 0.3.

Sy, Se Hiong, Mechanical management of aquatic vegetation. Ph.D. Thesis, Wisconsin 1974 Univ., U.S.A., 254 p. (Cited in Weed Abstr., 25(10):3023 (1976))

Analyses of Myriophyllum spicatum showed peak crude protein content only slightly lower than that of lucerne. Calcium and xanthophyll contents were high, and crude fibre low, in relation to forage crops.

Taylor, K.G. and R.C. Robbins, 1968. The amino acid composition of water hyacinth (Eichhornia crassipes) and its value as a protein supplement. Hyacinth Control J., 7:24–5

Protein and amino acid contents of water hyacinth were analysed.

Analysis (% Dry weight)
 Dry matterAshNCrude proteinCrude fibreEther extractN-free extract
Whole plant8.9-1.59.6---

Amino Acid Content: % of Water Hyacinth Crude Protein (16 g N/100g crude protein)

 % %
Lysine5.34Aspartic acid17.37
Isoleucine4.32Glutamic acid9.29

The recovery of amino acids accounted for 92.3% of the total protein. Losses may be due to either incomplete hydrolysis or destruction of other amino acids, including tryptophan.

The lycine content was found to be in sufficient concentration to serve as an effective supplement to grain protein.

The amino acid analyses were taken from a plant containing 9.6% protein. Higher protein levels occurred in June and July with lower levels in May, August and September.

Young plants have a higher protein content than mature plants but produce a proportionally smaller yield.

Taylor, K.G., R.P. Bates and R.C. Robbins, 1971 Extraction of protein from water hyacinth. Hyacinth Control J., 9(1):20–2

Water hyacinth analyses showed water content was 94.6%, and crude protein content was 0.43% fresh weight.

Analyses (% dry weight) at different seasons for crude protein and fibre:

Crude protein4.75.89.2

Telitchenko, M.M., G.V. Tsytsarin and Ye.L. Shirokova, 1970 Trace elements and algal “bloom”. Hydrobiol.J., 6(6):1–6

The literature from U.S.S.R. on the trace element content of algae and diatoms is reviewed.

The needs of algae for various trace elements and the concentrations at which these elements can prove toxic were determined. During a “bloom” of algae (Aphanizomenon flos-aquae and Microcystis aeruginosa) the following range of trace element concentrations were determined in 1 kg of algae:

μg × 102μg × 10g
MnTi, Ba, CuSn, Ni, Cr, Pb

* Welch, P.S., 1935 Chemical composition of aquatic plants. In Limnology, by P.S. Welch. New York, McGraw Hill, pp.280–1

The author has condensed aquatic plant analyses taken from papers by Scheutte and Alder (1927) and Scheutte and Hoffman(1921).

Analyses %)
ConstituentCladophoraMyriophyllumVallisneriaPotamogetonCastalia odorataNajas flexilisChara
Crude protein (N×6.25)18.1918.7511.808.0217.3811.624.50
Ether extract2.002.440.730.912.541.630.76
Crude fibre17.3315.0114.0018.8519.7018.419.32
Nitrogen-free extract26.8535.3841 4150.3037.2240.2339.50
Composition of the Ash (%)
ConstituentCladophoraMyriophyllumVallisneriaPotamogetonCastalia odorataNajas flexilisChara

* Williams, R.H., 1956 Salvinia auriculata Aubl.: the chemical eradication of a serious aquatic weed in Ceylon. Trop.Agric., 33(2):145–57

Analysis of Salvinia auriculata (= molesta):

 Dry weight %
Fresh weight % Water Organic matterAsh and sand NK2OP2O5CaO

* Anon., 1951 Water hyacinth - a new use. Madras Agric.J., 38(1):27–8

The estimated yield of water hyacinth per annum was 60 t/acre (150 tonnes/ha.)

The reader who has studied the analytical data given in this chapter, especially if the original papers have also been consulted, will have noted the considerable variation in results between authors working on the same plants and also within the work of one author or team. Many workers have drawn attention to this and have reported how variations can also occur between plants of different age and when sampled at different times of the year. Yet throughout there seems to be a consistent finding that the analyses of water plants when taken on a dry matter basis do not differ substantially, in organic or inorganic content, from typical crop and other plants growing on dry land.

A characteristic and different feature of some water plants which can complicate analytical results is the encrustation of lime, iron and other minerals on the surfaces of submerged plants as a physical deposit, or as a consequence of micro-organisms living on the plant surface. Rooted aquatic plants may, during harvesting, be brought to the surface with bottom mud entangled in their roots, again complicating analyses.

However, the main analytical difference between water and land plants is the high water content of the aquatics, because they are necessarily harvested wet, and have a higher internal water content. It is the inconvenience, and extra cost involved, of this feature which is a main limiting factor in the harvesting and utilization of aquatic plants.

More work to overcome this problem is needed to supplement the use of solar heat for drying, and mechanical pressing of water from the plant tissues. Both are included in some of the foregoing papers. However, in using any mechanical method of extracting water note should be taken of observations by N.W. Pirie (1971) (p. 135) that “it is difficult to press fluid out of an undamaged leaf”, and, if the plant is pulped before pressing, “protein is lost to an extent approximately proportional to the extent to which water is being removed.”

This harvesting problem is therefore not simple, but it must be faced and solved for widespread aquatic weed harvesting to be accepted (see Chapter IV).

Paddy growing on floating island in Rawa Pening lake, Central Java, Indonesia

Cyperus papyrus, Salvinia sp. and Pistia sp. in Lake Naivasha, Kenya

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