THE BOOT. 17 



In general terms it may be stated that the form assumed by the roots, 

 whether true or adventitious, is in direct relation to the nature of the 

 medium in which they grow and the purposes they have to serve as feed- 

 ing roots, hold-fasts ; or reservoirs of nutriment. 



Adventitious Roots (figs. 4, 7) are specially characteristic of , though 

 by no means confined to, Monocotyledons and Elowerless plants, 

 since their radicles are usually arrested in their growth ; they are 

 also necessarily the only kind which can occur upon specimens of 

 Dicotyledonous plants which have been raised, not from seeds, but 

 from cuttings, layers, tubers, &c. They arise from the side of the 

 stem which gives birth to them, and most readily in the vicinity of 

 buds or leaves. 



Adventitious roots are very Fig. 8. 



variable in form and consistence. 

 They may be fibrous (fig. 7) or 

 tuberous (fig. 8), and are not un- 

 commonly of intermediate charac- 

 ter in the Monocotyledons, con- 

 sisting of more or less thick fleshy 

 fibres. Either the fibrous or tube- 

 rous form may occur exclusively 

 in groups of adventitious roots, 

 or such groups may contain roots 

 or rootlets of both kinds. In 

 arborescent Monocotyledons the 



adventitious roots aCO Uire a WOody Fasciculate adventitious roots of Ranunculus 

 , j . i J Ficaria, partly fibrous, partly tuberous. 



character and great size ; in her- 

 baceous Monocotyledons they are commonly annual, or, if tuberous, 

 biennial. 



The fibrous adventitious roots of Monocotyledons are generally soft, 

 much elongated, and little divided, like those "at the base of bulbs of the 

 Hyacinth, Onion, &c. (fig. 17). A mixture of fibrous and tuberous adven- 

 titious roots, forming what is called a fasciculate root, occurs in Hemero- 

 callis, and in Ranunculus Ficaria (fig. 8), in which, as in the plant last 

 mentioned, the structure is still further complicated by the existence of 

 buds, as explained further on under the head of Tubers. A peculiar modifi- 

 cation of this structure is found also in most terrestrial Orchids. In Spircea 

 filipendula the fibrous roots exhibit tuberous thickenings at intervals. 



Root-hairs. The youngest parts of rootlets, whether branches 

 of axial roots or adventitious roots, often exhibit a coat of delicate 

 cottony root-hairs, which are thread-like growths from the epi- 

 dermis (tricliomes\ and are thrown off in perennial roots when the 

 epidermis gives place to the rind. 







