Chapter VI — 61 — Intracellular Distribution 



dominates and most of the cell's water is held as part of the living substance. In cells 

 that have lost their living function and possess little more than a cell wall skeleton, 

 such as vessel segments, tracheids, and sclereids, water may be present only in the im- 

 bibed state in the wall ; it may fill the lumen or not, depending upon the condition of 

 the tissue. These are some of the extreme variations ; others will be mentioned later. 

 While the cell is usually described as the unit of plant structure and function, the 

 so-called building block, it is becoming increasingly evident that caution is demanded 

 in such usage. The classical cell theory is being replaced by the concept that the 

 organism as a whole is the important unit, and that the interconnected protoplasts con- 

 stitute an integrated mechanism representing a higher order of organization than any 

 summation of the individual parts. This concept has been used by Munch (1930) in 

 consideration of the mechanism of solute movement in plants, his term for the inter- 

 connected protoplasm being the symplast as contrasted with the apoplast — the sum 

 total of non-living substances of the plant. This change has resulted from the realiza- 

 tion that a plant cell cannot be considered an independent entity but rather that it is a 

 highly coordinated part of the whole (symplast), connected to adjacent cells by proto- 

 plasmic .strands and also by continuous water. At the same time, in spite of its inter- 

 connections, each cell maintains a certain degree of individuality. The use of cells as 

 functional units in experimental work (plasmolysis, etc.) is a matter of convenience. 

 The reliability of the results of such work often depends to a large extent upon the 

 degree of injury to the cells from isolation of the tissue. 



Water Content: — Water is included among tlie components of all 

 plants but the variation in composition is so great that it is impossible to 

 give general figures for the content. One example that might be considered 

 typical of many field crops is corn, which Latshaw and Miller (1924) 

 found to contain 71.4 per cent water when harvested at the time the grain 

 matured. Many forage crops contain 75 to 80 per cent moisture when cut 

 for hay, and fruit and vegetable crops contain even more water. The water 

 content of plants usually shows a periodic diurnal variation and a progres- 

 sive decrease with maturity. Pisek and Cartellieri (1931, 1932) found 

 the water content in the leaves of a number of herbaceous mountain plants 

 growing in full sun to be 69 to 83 per cent of the fresh weight ; for cer- 

 tain shade plants it was greater, 80 to 86 per cent. Leaves of trees gen- 

 erally contain less water than those of herbaceous species. The authors 

 just cited reported (1939) a range from 50 to 70 per cent (of fresh weight) 

 for youngest and oldest leaves of Fagus sylvatica. Water in the trunks 

 of trees is lower in amount than in leaves. Gibbs (1935) gives figures 

 varying in the range 40 to 55 per cent of wet weight for trunks of quaking 

 aspen. 



At the lower extreme of water content, some of the simpler plants can 

 survive almost complete desiccation during long dry periods. Scofield 

 and Yarman (1943) report values for the lichen Umhilicaria as low as 6.1 

 per cent of the fresh weight ; for the terrestrial alga Pleurococcus, Fritsch 

 (1922) found a lower limit of about 5 per cent. 



A general idea as to the water content of certain plant organs used as 

 food materials may be had from Table 19. 



Binding Forces : — The absorption and retention of water by plant 

 cells depend upon forces that have been, for convenience, termed osmotic, 

 imbibitional, and chemical. Whether or not there is any fundamental 

 difiference between them, the net effect of all these forces, including ionic, 

 covalent, dipole, and hydrogen bonds, microcapillary forces, and the col- 

 ligative effect of solutes, is to lower the free energy and hence the diffusion 

 pressure of water. 



Osmotic pressure has been treated in detail in the two preceding chap- 

 ters. As previously stated, the addition of a solute to a solvent causes an 



