50 PLANTS 



springs from the stem above ground, or even, in some cases, 

 from the branches, and grows down to and into the soil. Such 

 roots are found in special cases in which the plant would other- 

 wise be top-heavy for its basal root system. The prop roots 

 are primarily for anchorage though they may also serve for 

 absorption. 



105. Another form of modified root is found in certain climb- 

 ing plants which have roots springing from the aerial parts of 

 the plants. These aerial roots serve as hold-fasts, penetrating 

 the superficial layers of the bark of the tree, or crevices of the 

 rock, to which the plant clings for support. 



106. The function of the aerial roots of epiphytes, so common 

 in humid climates, is not only to attach the plant to its host 

 but also to absorb moisture. In some cases the moisture is 

 absorbed directly from the atmosphere; in others, it is drawn 

 from the sponge of decaying leaves and other vegetable sub- 

 stance which collects among the tangled mass of roots. In the 

 latter case the absorbed water is likely to contain more or less 

 nourishing matter, extracted from the humus. 



Modified Stems and Branches 



107. Stems are frequently so much reduced in length that 

 the leaves seem to spring directly from the roots. In such 

 ''stemless plants," however, the conical or disc-shaped surface 

 from which the leaves arise must be regarded as the stem, at the 

 apex or centre of which the terminal bud will always be found. 

 Many biennials remain ''stemless" during the first season, but 

 during the second period of growth produce a normal stem 

 and branch system by development from the terminal bud. 



108. Another type of stem is that characteristic of the climb- 

 ing and trailing plants. In these the stem is too slender to 

 maintain itself in an erect position. The climbers depend on 

 other objects for support, the stem serving merely as the 

 conducting system connecting roots and leaves. In the 



