586 - Heredity and Evolution 



Fig. 30-11. This cactus is one of the extreme xerophytes (see text). (Courtesy of 

 C. J. Alexopaulos.) 



(for example, certain cacti) the leaves are re- and roots display a relatively simple struc- 

 duced to mere spiny structures (Fig. 30-11), ture. In Elodea, for example, the roots are 

 and the compact fleshy stem takes over the scarcely more complex than holdfasts (p. 

 photosynthetic functions of the leaves. Other 

 xerophytes, however, possess thick fleshy 

 leaves in which large reserves of water may 



238), and the stem and leaf cells absorb 

 water directly from the surroundings. More- 



be accumulated. 



The mesophytes are plants adapted to con- 



over, the vascular and strengthening tissues, 

 especially of the xylem, are almost lacking 

 in the Elodea, and since all the cells possess 



ditions of moderate moisture. This group chloroplasts, there is no differentiaton be- 



includes a number of very quick-growing 

 annual plants that can extend their range 

 into desert regions if a small amount of rain- 

 fall occurs each season. For a few weeks each 

 year such deserts may display a rich variety 



tween the epidermis and chlorenchyma. 



Other hydrophytes (for example, certain 

 water lilies) have underwater roots and stems, 

 but the leaves and flowers are exposed to the 

 atmosphere. In such cases, usually, only the 



of bright-colored flowers, which mature their aquatic parts are simplified. And finally, in 

 seeds very quickly. Then when the drought swamp-dwelling hydrophytes, only the roots 

 sets in again, the seeds lie dormant until the may be submerged. Such roots are not apt to 

 next rainy period begins. However, there is be highly modified, except that aerial 

 an exceedingly wide variety of mesophytes, branches, called pneumatophores (Fig. 30- 

 which, in fact, include a majority of familiar 12), are characteristic, especially in the case 

 plants in temperate regions. of large-deep-rooted swamp trees. Probably 

 The hydrophytes include all higher plants the pneumatophores fulfill a respiratory func- 

 that live in regions where there is an over- tion, being designed to transmit air down- 

 abundance of water, and hydrophytes are ward through porous channels to the deeper 

 variously adapted to different local condi- parts of the root system — an important func- 

 tions. Some species, like Elodea, live entirely tion, since oxygen diffuses very slowly 

 under water, in which case the leaves, stems, through a swampy soil. 



