82 Plant Life and Evolution 



ficient moisture for their needs upon the surface of 

 the shaded ground or the sheltered sides of tree 

 trunks and walls. They readily absorb water from 

 the moist substratum or from the air, and this is 

 sufficient for their normal growth. A small num- 

 ber of algae, e.g., Botrydium (Fig. 7), regularly 



FIG. 7 



Botrydium, a terrestrial alga provided with roots for taking 

 up water. Much enlarged. 



grow entirely exposed to the air and obtain their 

 water from the earth by means of root-like organs, 

 thus behaving like genuine land plants; but owing 

 to their extreme delicacy the period of growth is 

 usually very brief, and as their cells are not ade- 

 quately protected against loss of water by evapora- 

 tion, they can only reach very small dimensions. 



