DEVELOPKMENT OF VEGETABLES. 81 



The roots differ considerably from the stems both 

 in their structure, and in their mode of growth. 

 They exhibit, indeed, the appearance of medullary 

 rays and of concentric layers, but they are destitute 

 of any central pith, and they have no trachete ; 

 neither does their surface present any appearance 

 of stomata. They increase in thickness in the same 

 way as the stem increases. This law obtains both 

 in exogenous and endogenous plants : they do not, 

 however, grow in length by the elongation of any 

 of their parts, but simply by additions made to their 

 extremities. Their ramifications are not the result 

 of the developement of buds, as are the branches 

 of the stem ; but they arise merely from the addi- 

 tional deposits taking different directions. Almost 

 every part of the surface of the stem or branches may 

 shoot forth roots if they are covered with earth, and 

 properly moistened, and if they are supplied with 

 sap from the circulating system of the plant itself. 

 De Candolle asserts that they generally grow from 

 certain points on the surface of the bark, which ap- 

 pear as dark spots, and which he terms Lenticellce .^ 

 Great variety exists in the form and disposition of 

 roots in different families of plants, according to the 

 particular purposes they are intended to serve, con- 

 formably with their general functions of absorption 

 and of mechanical support. Both these purposes 

 are promoted by their sending out from their sides 

 numerous fibrils, or lesser roots, which increase 

 their firm hold upon the soil, as well as multiply 

 the channels for the introduction of nourishment. 



* Ann. Sc. Nat. vii. i., and Organographie, i. 94. Professor 

 Hugo Moll], on the other hand, contendc, that this opinion is unsup- 

 ported by fact. Ibid. serie2, Bot. x. 33 and 46. 

 VOL. I. G 



