THE ANGIOSPERMAE : ROOTS 



827 



which usually arise endogenously in the same manner as branch roots. Only 

 in Linoria are the root buds exogenous. A few cases are known in which 



Conical root 



Fig. 827. — Tillandsia recwvata. Transverse section 

 of stem showing numerous internal roots. 



even the root apex may become transformed into a bud, namely in the 

 Monocotyledons L/^^^rfl and A^^o^^m (Orchidaceae) and in Anthiirium(Arzce^e). 

 This formation of adventitious buds may be divided into three categories 

 on biological grounds. 



1. Reparative. Buds formed as a sequel to root injuries or to the destruc- 

 tion of the normal shoot system. This is common in woody plants and is 

 often additional to the formation of buds of the next category. Among 

 herbs there are fewer examples, and in most herbaceous plants where 

 reparative buds are formed there are no others on the roots, e.g., Crambe, 

 Cochlearia, Anchusa and Morisia. They form a valuable means of vegetative 

 propagation. 



2. Accessory. Buds formed as part of the normal development in addition 

 to the usual stem buds. They may serve as a means of vegetative propaga- 

 tion and are often called " suckers." They are common among woody 

 plants, e.g., Popnliis, Riibiis, RJnis, Primus, Hydrangea. They are rare among 

 herbs. Well-known examples, however, are Riimex acetosella and Epilobium 

 angiistifolium and certain other aggressive weeds. 



3. Necessary. These are buds which are the plant's only means of 

 survival, normal buds being abortive or lacking. The saprophytes Pyrola 

 and Monotropa are in this category and here also may be classed Lepidium 

 latifoliwn and Armoracia vidgaris (Horse Radish) since they scarcely ever 

 set good seed. 



