.MORPHOLOGY 



47 



surrounding and overlying tissue of the parent root, by the ruptured 

 portions of which they are often invested at the base, as with a collar. 



ADVENTITIOUS HOOTS, just as adventitious shoots, may arise from 

 any part of a plant. They are especially numerous on the under side 

 of rhizomes (Fig. 23 w), and also, when the external conditions are 

 at all favourable, they seem to develop very readily from the stem 

 nodes. A young shoot, or a cutting planted in moist soil, quickly 

 forms adventitious roots, and 

 roots may also arise in a similar 

 manner from the bases of 

 leaves, especially from Begonia 

 leaves when planted in soil. 

 The origin of adventitious roots, 

 as of almost all roots, is en- 

 dogenous. Dormant root rudi- 

 ments occur in the same manner 

 as dormant buds of shoots. 

 Willow-twigs afford a special 

 case of the presence of such dor- 

 mant rudiments of adventitious 

 roots, the further development 

 of which is easily induced by 

 darkness and moisture. 



When adventitious shoots 

 are borne upon roots they 

 mostly arise at some distance 

 from the apex. In the Adder's 

 Tongue Feru (Ophioglossum), the vegetative reproduction of which is 

 effected by means of buds formed on the root, these arise close to the 

 root-apex. Lastly, just as terminal leaves may arise from the apex of 

 a stem and terminal shoots from the tips of the leaves of Ferns, in a 

 few instances (species of Asplenium and Platycerium) a shoot may arise 

 as the direct continuation of the growing point of a root ( 2r> ). 



The Form of Subterranean Roots. The customary nomenclature 

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

 branching. A root which 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 

 Gymnosperms 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 

 which serve for storage of reserve materials may enlarge and become 

 turnip-shaped or tuberous (Fig. 51). Such tuberous growths often 

 greatly resemble stem tubers, but may be distinguished from them 



Fin. 51. Root-tubers of iJahlia rariabilis. 

 lower portions of tin- cut stems. (! nat. 



The 



