DEVELOPEMENT OF VEGETABLES. 93 



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 tracheae ; neither does their surface 

 present any appearance of stomata. They in- 

 crease 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 additional deposits taking different di- 

 rections. 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 cir- 

 culating system of the plant itself. It is ob- 

 served, however, that they generally grow from 

 certain points on the surface of the bark, which 

 appear as dark spots, and are termed LenticeUo'.'^ 

 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, conformably to their general functions of 



* This name was given to them by De Candolle, Annales des 

 Sciences Naturalles, VII, I. and Organographie, I, 94. 



