164 BOTANY. 



211. The formation and development of a new root is 

 interesting and suggestive. It usually takes place at some 

 distance from the primary meristem, in the cambium or peri- 

 cambium. In the root of Trapa natans it takes place as fol- 

 lows : The cells of a restricted portion of the pericambium 

 divide by tangential walls into an outer layer, which becomes 

 the dermatogen of the new root (d, Fig. 139), and an inner 

 layer, from which develops its primary meristem (n, Fig. 

 139). The inner cells multiply by divisions in several direc- 

 tions, and as their mass increases they push out the young 

 dermatogen (B, (7, and D, Fig. 139). From the dermato- 

 gen the first layer of the root-cap is formed by the tangen- 

 tial division of its cells (C, li, Fig. 139). These growing 

 tissues push out the overlying portions of the mother-root, 

 and finally break through them. The root is thus seen to 

 be a strictly endogenous formation ; there is no connection 

 between its tissues and the epidermal and cortical portions 

 of the mother- root, the sole connection being with the deep- 

 lying tissues in, or in connection with, the fibro-vascular 

 bundles. Herein roots present a marked contrast to stems 

 and leaves, which, as a rule, develop from the exterior of 

 the plant-body, or, in other words, are exogenous in their 

 origin. 



212. Boots are rarely arranged in as regular an order as 

 are stems. In general they arise in acropetal order upon the 

 mother-roots of Pteridophytes and the primary roots of Pha- 

 nerogams, but this order is subject to many more disturbing 

 influences than in the case of the origin of stems. As to 

 position, they may arise in rows or ranks, or in particular 

 spots, dependent upon the disposition of the fibro-vascular 

 bundles, or the generating tissues in the root or stem. Thus 

 it may happen that on a root or stem there may be as many 

 rows of roots as there are fibro-vascular bundles. Roots 

 which develop from stems are generally much more affected 

 by external influences than those which grow from othei 

 roots. The degree of moisture of the different parts of the 

 stem appears to have much to do in determining the point 

 of the appearance of roots ; this is seen in stems which touch 

 the ground, as in the tomato, and in climbing plants, as the 



