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 

 I i JO| the plant. The entry of water into a cell 



I I (is * s tnrou &h tne P rocess called osmosis. 



I I IJ iff The plasma membrane of the cell is a 



ibsf kagjl semipermeable membrane which is almost 



FIQ. 38. A tur- perfectly permeable to water but almost 



moiyzecuefi. 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 



