VEGETATION TYPES. 49 ! 



face. Some of the blue-green algas develop on the mud in the 

 bottom of pools, and the accumulation of gases in the tangle later 

 buoys them up to the surface of the water. 



3. Provision jor aeration. Aeration is provided for through 

 the abundant air-spaces in the more bulky stems and leaves, and 

 also by the ability of many plants to float to the surface where 

 the threads of algae are, or near the surface, or in the case of 



Fig. 490. 



Swamp forest on the Gulf Coast. Bald cypress covered with hanging moss 

 (Tillandsia). In the foreground floating plants, the water hyacinth. 



floating leaves the upper surface is in touch with the air. In 

 the latter case there is a great development of stomata in the 

 upper surface of the leaf. 



4. Provision jor distribution oj food. The epidermal cells, as 

 well as the tissues of most water plants, are provided with thin 

 walls where they are in contact with water. This permits osmo- 

 sis to take place readily between the surrounding water, which 

 contains dilute food solutions, and the protoplasm in the cells. 

 Owing to the loose character of the tissues in the stems and 

 leaves, diffusion from the surface throughout the tissues is not 

 difficult. 



