478 On the Structure of Roots. 



dition of the roots is chiefly met with in biennial or perennial 

 herbaceous plants, such as do not flower in the season in which 

 they are raised from seed, and do not produce any deveh)pment 

 of the ascending stem sufficient to provide space for the assimi- 

 lated nourishment which is always laid up in store by plants 

 previously to their entering into a resting stage. The more 

 highly the yearling plants are fed the more nourishment they 

 will have to store up, and hence the tuberous enlargement of 

 these roots under cultivation.* The internal alterations which 

 produce these changes will be carefully examined when we are 

 examining the structure of roots. 



If the plant is a woody perennial, a tree or shrub, the main 

 root merely undergoes enlargement and solidification, accom- 

 panied by a more or less considerable development of branches, 

 corresponding to the expansion of the stem or trunk. 



In many perennial herbs the tap-root, developed at first, is 

 soon lost sight of in a bunch of fibrous roots, equalling it in size, 

 and of similar structure. For instance, in the groundsel, while 

 the original root is penetrating downwards, and sending out 

 branches as it grows, other and larger branches arise as adven- 

 titious roots from the older, upper part of the root, and from the 

 base of the stem, which new roots ultimately rival in size the 

 first, and formerly main, root, P'ibrous tufts of roots of this kind 

 are found at the base of many perennial herbs raised from seed, 

 and they exactly resemble tlie fibrous tufts of roots which are 

 formed by the same plants when they are propagated by cuttings, 

 layers, offsets, &c. Most frequently these roots are of fleshy 

 character and of short duration, being mostly developed late in 

 one season, and, after officiating for the support of the next 

 spring growth, dying away, to be succeeded by others connected 

 with the buds provided for the next succession of developments. 

 The strawberry furnishes a common example of this : when the 

 plants have borne fruit, and are sending out runners, some of the 

 axillary buds remain in close connexion with the old structure, 

 and form short prolongations of the stem (rootstock), on which 

 are developed a number of new adventitious roots, while the old 

 ones on the hinder woody part of the stem die away. In a con- 

 siderable number of Dycotyledonous herbs of this kind, a portion 

 of these fibrous roots even become tuberous, to lay up a store for 

 future growth, and this is chiefly the case in such plants as die 

 down below tlie soil annually, and are at the same time devoid 

 of a fleshy or woody rootstock ; the stem-structure all disap- 



* It must not be forgotten, that particular seasons, or other conditions, may 

 stimulate these biennials to flower in the first season instead of storing up their 

 nutriment in their roots. 



