4 8 



ELEMENTARY BIOLOGY 



years, and to give protection to other kinds of plants until the 

 earth has become compact (Fig. 12). 



Although roots do not generally put forth buds or shoots, 

 the roots of some shrubs and trees as certain willows, 

 poplars, and hawthorns do so, and can be used for propa- 

 gating the species. In some plants the roots will form new 

 shoots if the old shoot is completely removed or destroyed. 



On the other hand, roots 

 frequently arise from stems 

 or leaves, thus making pos- 

 sible the propagation of 

 plants by means of cuttings. 

 Roots that originate in this 

 manner are called adventi- 

 tious roots. Most of our 

 common house plants, and 

 willows and other trees, can 

 be propagated by keeping 

 twigs in water or wet sand 

 until roots appear, and then 

 transplanting them into soil. 

 If the leaf (or even a piece 

 of leaf) of a begonia or of 

 a bryophyllum be placed on 

 damp earth or sand, tiny 

 roots will be seen growing from various points along the edge 

 in the course of a few days. In these species buds will also 

 be produced, so that after a while we can separate small but 

 complete plants from the leaf, and get these to grow into 

 full-sized individuals (Fig. 13). 



Blackberry and raspberry bushes are frequently propagated 

 by layering, which consists in bending the flexible stems out 

 and burying the tips in the ground. Adventitious roots are 

 formed on the covered portions, and, later, buds form new 

 shoots. The old connecting stem is then cut away. A similar 



FIG. 15. Climbing roots 



The English ivy, like many other climbing 

 plants, clings to its support by means of adven- 

 titious roots that grow out all along the stem 



