PLANT SOCIETIES 



243 



because they absorb their nourishment through all parts 



of the epidermis directly from the medium in which they 



live. That they may absorb 



readily, the tissues are apt to 



be soft and succulent and the 



walls of the cells composing 



them very thin. In some of 



the pipeworts (Eriocaulons), the 



cells are so large as to be 



easily seen with the unaided 



eye. If you can obtain one 



of these, examine it with a lens 466.— Transverse section through 



and notice how very thin the ''^l, T"^ f. ^ hyd'-ophyte plant 



-' \Elatine ahmastruin), showing the 



walls are. Water plants also very large air cavities (GOODALE, 



contain numerous air cavities, 



after REINKe). 



and often develop bladders and floats, as in the common 



bladderwort, and many seaweeds (Fig. 467). 



Swamp plants, drawing their nourishmeitt from the 



loose soil in which they are anchored, and lacking the 

 support of a liquid medium, develop 

 roots and vascular stems. The roots 



467. — Seaweed (sar- 

 gassum) with bladderlike 

 floats. 



468. — A cypress trunk, showing enlarged base for 

 aeration. 



of plants growing in swamps have difficulty in obtaining 

 proper aeration on account of the water, which shuts off 

 the air from them, hence they are furnished with large 



