Grafts et al. — 62 — Water in Plants 



Table 19. — Water content, in per cent of fresh weight, of some common food materials 



(data of Chatfield and Adams, 1940) : — 



Water content 

 as per cent of 

 Plant fresh weight 



Lettuce (inner leaves) 94.8 



Tomato (red, ripe fruit) 94.1 



Radish (edible portion root) 93.6 



Cabbage (edible portion) 92.4 



Watermelon (fruit flesh) 92.1 



Carrot (edible portion root) 88.2 



Onion (edible portion bulb) 87.5 



Apple (edible portion fruit) 84.1 



Potato (whole tuber) 77.8 



Prune (fresh, flesh and skin) 76.5 



Field corn (dry whole grain) 110 



Common bean (dry seeds) 10.5 



Barley (hulless type grain) 10.2 



Peanut (raw, with skin) 5.1 



increase in the average distance between the solvent molecules and hence a 

 lowering of the diffusion pressure of the solvent. Forces of attraction be- 

 tween solute and solvent molecules cause further diffusion pressure lower- 

 ing. Any such lowering measurable by separating the solution from the 

 pure solvent by a membrane permeable only to the solvent will be consid- 

 ered as due to osmotic force. 



Imbibitional forces result from the attraction between solvent molecules 

 and colloidal substances. Their magnitude may be such that the diffusion 

 pressure of the imbibed water is lowered to the extent of hundreds of atmos- 

 pheres. Most plant colloids are of the hydrophilic type, that is, they have 

 a strong attraction for water. Because of their structure, colloids are char- 

 acterized by a high surface : mass relation and surface energy undoubtedly 

 enters into the binding or immobilization of water molecules. 



The absorption of water by a hydrophilic colloid results in swelling, 

 and pressures of great magnitude may be exerted by swelling seeds, wood, 

 etc. Imbibitional forces are of great importance in the retention of water 

 by cell wall substances. The initial pull developed as a result of evapora- 

 tion of water from the mesophyll of leaves is imbibitional in nature and only 

 after transmission of this pull or deficit through the continuous aqueous 

 phase of cell walls, protoplasm, and vacuole do the osmotic and hydrostatic 

 aspects of transpiration pull become manifest. 



By chemically bound water is meant that water which enters into true 

 hydrate formation. Depending upon the nature of the substances involved, 

 this water has also been termed water of constitution and water of crystal- 

 lization. In dealing with cellulose and similar colloidal materials the con- 

 cept of chemically bound water may be difffcult to visualize. Such water 

 we understand to be that which differs from surface bound water only in 

 that it combines in definite proportion to the bonding material as if it were 

 localized in and limited by the lattice structure. 



Water of Cell Walls : — The water retaining function of the plant cell 

 wall is closely tied up with its chemical and physical makeup. The wall is 

 pictured as consisting of three layers : 



1 ) The middle lamella, or intercellular substance. This consists almost 

 entirely of pectins and lignins deposited by the cytoplasm. 



