MORPHOLOGY 



161 



youngest leaves, and consequently at points where the differentiation 

 of the surrounding tissue has 

 already begun ( 156 ). 



The vegetative cone, in the 

 case of strictly dichotomously 

 branching shoots (cf. p. 19), in- 

 creases the number of its cells 

 in the direction of the plane of 

 the subsequent bifurcation, and 

 eventually gives rise to two new 

 growing points. 



With the exception of those 

 Pteridophytes whose roots as 



FIG. 169. Thuja, nccidentalis. A, Median longitudinal 

 section through the ripe seed (x .5). B, C (x 2) ; 

 D, E (nat. size), different stages of germina- 

 tion ; h, hypocotyl ; c, cotyledons ; r, radicle ; 

 v, vegetative cone of stem. 



well as stems are dichotomously branched 

 (cf. p. 46), the branches of all other normal 

 roots, whether growing by means of an apical 

 cell or not, arise in acropetal succession ; the 

 branching first begins in regions considerably 

 removed from the growing point, where the 

 differentiation of the tissues is already com- 

 plete. In Phanerogams new roots are developed 

 in the pericycle : in Pteridophytes in the inner- 

 most cortical layer. The lateral roots must 

 consequently push through the whole cortical 

 layer of the parent root. They are situated 

 either directly opposite to the vascular strands 

 of the parent root, or between the xylem and 

 phloem strands. The number of rows of lateral 

 roots is, therefore, as VAN TIEGHEM ( 157 ) 

 pointed out, either equal to or double the 

 number of vascular strands. As the strands 

 of the vascular bundles of roots take a straight 

 course, the lateral roots must similarly form 

 straight rows. The distances between the rows 

 themselves are either equal, or, when the lateral 

 roots are situated to the right and left of each 

 vascular strand, the rows are arranged in 

 pairs with wider intervals between each pair. 



M 



Fio. 170. Seedling of CVi^.//. tu 

 Jietulits. h, Hypocotyl ; c, 

 cotyledons ; hu; main root ; 

 sw, lateral roots ; r, root- 

 hairs; c, epicotyl; I, I', foliage 

 leaves. (Nat. si/e.) 



