STUDIES OF SEED-PLANTS 159 



greenhouse plants from slips or cuttings of stems which 

 form such roots when set in moist sand. In order to prevent 

 erosion along river banks, stakes cut from branches of willows 

 are often driven into the soil and soon form adventitious 

 roots and put forth leaves, forming young trees. Straw- 

 berry runners (which are branches of stems trailing along 

 the ground) , blackcap raspberry stems, and numerous common 

 weeds " take root"; that is, form adventitious roots at the 

 nodes. Bryophyllum leaves and those of some begonias 

 form roots when placed on moist soil, and develop new 

 plants. 



152. Root-tubercles. These were mentioned in connec- 

 tion with the bean root ( 67). Only by the aid of the bac- 

 teria which live in the root-tubercles is it possible for plants 

 to make use of the nitrogen which constitutes over two 

 thirds of the air. These bacteria have the peculiar power 

 of combining nitrogen with other elements derived from the 

 soil, and the nitrogen compounds thus formed are absorbed 

 by the roots and used by the plant just as it uses the nitrogen 

 in manures and in such soil fertilizers as nitrate of soda. 



153. Special Adaptations of Roots. The great majority 

 of plants have roots whose functions are (1) anchoring the 

 plant, and (2) absorbing water and dissolved substances 

 from the soil ; but some roots have interesting modifica- 

 tions of structure to adapt them to other kinds of work. 

 Such modified structures which are fitted to special work or 

 functions are called adaptations. Some of the most im- 

 portant of such specialized root structures are aerial roots, 

 prop-roots, water roots, breathing roots, and parasitic roots. 

 These will be described in this order. 



(1) Aerial Roots. Many plants develop roots in the air; 

 that is, above the soil. Such roots may serve as clinging 

 organs, as in poison ivy, trumpet-creeper, and English ivy ; 

 and on some plants they are important absorbers of water. 

 Many so-called " air-plants," from tropical countries, such 



