CELLS AND PROTOPLASM 



75 



vacuole or many. (See Figure 26-) Vacuoles are spaces 

 filled with sap. In young cells the protoplast fills the 

 interior, but as the cells enlarge much of the interior comes 

 to be filled with sap. Sap is the most fluid part of plants ; 

 it is simply water with various sub- 

 stances dissolved in it. You may 

 be familiar with the sap of maple 

 trees whose sweetness is due to the 

 sugar dissolved in it. 



Sap enters cells more freely than 

 it passes out of them. The proto- 

 plasm appears to encourage its en- 

 trance and to discourage its exit. 

 It enters living cells until there 

 comes to be a pressure outwards, 

 like the pressure of gas against the 

 inner walls of a balloon. The walls 

 of active cells are elastic and they 

 tend to curve out as the sap presses 

 against them. The cells press upon 

 each other, thereby helping make 

 the whole plant body rigid. This swollen state of the cells 

 is called turgidity. (See Figure 2/.) The cells of a fresh 

 leaf are turgid. The cells of a wilted leaf have lost their 

 turgidity. Water has evaporated from them more rapidly 

 than it was replaced. 



Plant cells when mature have many different forms and 

 functions, but when young they are all much alike. They 

 become different as they grow. This process of becoming 

 different is called differentiation. Mature cells may be 

 very dissimilar in appearance, as the pictures show. Yet 

 there are certain life processes which occur in all living 



Fig. 26. — A few cells from a 

 leaf of moss, showing nuclei, 

 chloroplasts, cytoplasm, and 

 scattered vacuoles. 



