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 

 semipermeable membrane which is almost 

 Fia. 38.— A tur- perfectly permeable to water but almost 

 Soiyz''ed'*cefi. '''^^' 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 



