72 PLANT PHYSIOLOGY 
even greater amounts. Fleshy fruits like the pear and 
grape contain still larger amounts. Algae are extremely 
watery, the amount of water in Spirogyra probably ex- 
ceeding 97 per cent. This water is present not only 
in the vacuoles but also in the cell wall and protoplasm, 
both of which have the property of imbibing water to a 
considerable extent. ‘Thus even lignified cell walls may 
have one-third of their weight as water and protoplasm 
is probably not active unless 75 per cent. or more water 
is contained in it. 
105. This water is almost continuous throughout the 
whole plant, so that we may think of a plant as a mass 
of water of the shape of a plant with the interstices oc- 
cupied here by molecules of cell wall substance, there by 
protoplasm, the water being continuous also with the 
water surrounding the roots in ordinary plants, or the 
whole plant if it is aquatic. 
106. Although the water is continuous throughout 
the plant, it is held more abundantly in some parts than 
others, and may be in motion within 
the plant. The entry of water into a cell 
is through the process called osmosis. 
The plasma membrane of the cell is a 
Fig. 38.—A tur- perfectly permeable to water but almost 
ST oil: Plas- impervious to some of the substances in 
solution in the water of the cell. Under 
such circumstances, if the solutes inside the cell are more 
concentrated than those outside, the molecules of water 
pass more rapidly into than out from the cell and it 
becomes filled with water. The protoplasm is pressed 
against the cell wall and this stretches until it may be 
increased in area in some cases by as much as 50 per cent. 
This stretching continues until the wall can stretch no 
semipermeable membrane which is almost | 
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