786 



A TEXTBOOK OF THEORETICAL BOTANY 



Additional characteristics, usually found but not essential, are : A positive 

 or neutral reaction to gravity, i.e., a tendency to grow downwards or sideways 

 rather than upwards ; the absence of chlorophyll ; and unsusceptibility to the 

 influence of light. 



Of all the above characters the only one which appears to be fundamental 

 and invariable is leaflessness. No true root produces leaves, but some may 

 occasionally produce buds, which grow into leafy stems. On the other 

 hand there are also some true stems which are leafless. 



These facts show that even so fundamental a morphological category as 

 that of the root has no absolute value, since it cannot be rigidly defined. An 



Fig. 784. — Moneses grandifloro . Root system with temporary leafy shoots. 



{After Velanovsky.) 



organ can only be judged as to whether it be a root or not on the balance of 

 its characters, and while normal cases present no difliculty there are some 

 organs, like the rhizophores of Selaginella and the attachment organs of some 

 parasites, which are very difficult to classify. 



Roots and stems are by no means indispensable to each other. Roots, 

 for example, can be grown artificially as root cultures. A root apex placed 

 in a sterile culture solution will grow and branch, forming a system that fills 

 the culture vessel, without producing any stems. These root cultures can 

 be propagated vegetatively, through successive culture vessels, for long 

 periods, and form nothing but roots. Practically independent root systems 

 exist also in nature. In the saprophytic family of the Pyrolaceae certain 

 species, e.g., Moneses grandiflora (Fig. 784) and Monotropa hypopitys, produce 

 at germination nothing but a root, which grows and branches abundantly 



