48 



BOTANY 



TART I 



The Form of Subterranean Roots. — The customary nomenclature 

 for the various root forms is based on their shajoe, size, and mode of 

 branching. A root wliich is a prolongation downwards of the main 

 stem is called the main root or TAP-ROOT ; the other roots are termed, 

 with reference to the tap-root, lateral ROOTS of different orders, 

 according to the order of their development. A tap-root is present in 

 G3nnnosperms and many Dicotyledons, while it is, as a rule, wanting 

 in Monocotyledons. The root system of most Monocotyledons and 

 many Dicotyledons mainly consists of adventitious roots arising from 

 the base of the stem or from underground stems or rhizomes. Roots 



Fin. 51. — Root-tubers of Dahlia variabilis, s, The 

 lower portions of the cut stems. (\ nat. size.) 



Fig. 52.— Orchis latifolia. V, The 

 old root-tuber; t", the young 

 root-tuber ; 6, floral shoot ; s, 

 scale leaf with axillary bud, 

 k, from which the uew tuber 

 has arisen ; r, ordinary adventi- 

 tious roots, (i nat. size.) 



which serve for storage of reserve materials may enlarge and become 

 turnip .shaped or tuberous (Fig. 51). 8uch tuberous growths often 

 greatly resemble stem tubers, but may be distinguished from them 

 by their root-caps, by the absence of any indications of leaf develop- 

 ment, and by their internal structure. 



The morphology of the tubers of the Orchidaceae is peculiar. They are, to a 

 gi-eat extent, made up of a fleshy, swollen root terminating above in a shoot-bud (-^). 

 At their lower extremity the tubers are either simple or palmately segmented. 

 In the adjoining figure (Fig. 52) both an old {t') and a young tuber {t") are repre- 

 sented still united together. The older tuber has produced its flowering shoot {h), 

 and has begun to shrivel ; a bud, formed at the base of the shoot, in the axil of a 

 scale leaf (s), has already developed an adventitious root which has given rise to 

 the younger tuber. Roots of ordinary form arise from the base of the stem above 

 the tuber. 



Metamorphosis of Roots. — The aerial roots of tropical epiphytes, 

 plants living upon others often at a considerable height from the 



