138 



BOTANY 



PART 1 



is that each of the strands of xylem of the root rotates through 180 round its 

 longitudinal axis, bringing the protoxylem to the inner side of the strand which is 

 the characteristic position in the stem. -A number of collateral vascular bundles 

 are reconstituted from the tissues of the radial bundle of the root by the radially- 

 arranged xylem and phloem taking up the collateral position. This happens in 

 different ways, of which two main types may be distinguished : 1. The strands 

 of xylem when rotating follow a straight course from the root to the stem ; the 

 strands of phloem of the root, on the other hand, divide radially, the two halves 

 separate tangentially, and, uniting with the portions derived from adjoining 

 strands of phloem, come to lie outside the 

 xylem strands. 2. The phloem strands of 

 the root follow a straight course into the 

 stem, but the strands of xylem which rotate 

 through 180 split radially ; the halves 

 separate tangentially (as the phloem strands 



FIG. 164. Transverse section of the root 

 of Vicia Faba showing the origin of a 

 lateral root (r). e, Endodermis ; p, peri- 

 cycle ; d, cortex ; g, xylem strand ; v, 

 phloem strand of the radial bundle, 

 (x 40. Somewhat diagrammatic.) 



FIG. 165. Portion of a longitudinal section 

 of a root of Amarantus showing the origin 

 of a lateral root, e, Endodermis, already 

 absorbed opposite the young root; d, cor- 

 tex ; p, pericycle ; sp, spiral tracheide ; r, 

 young lateral root, (x about 200. After 



PH. VAN TlEGHEM.) 



did in Type 1) and, uniting with the portions derived from the adjoining strands 

 of xylem, place themselves internal to the strands of phloem to constitute the 

 collateral bundles. 



4. Branching of the Root. By this process, in which a root 

 always gives rise to roots, the root-system can penetrate the soil in 

 all directions and obtain from the whole space thus occupied water 

 and dissolved salts. 



DiCHOTOMOUS branching by an equal division of the growing 

 point only occurs in some Pteridopliyta (Lycopodinae). 



With this exception the branching of the root is LATERAL (Fig. 

 158), the lateral roots, in contrast to the lateral shoots, originating at 

 some distance from the growing point where the meristematic cells 

 have been transformed into permanent tissue. They arise ENDO- 

 GENOUSLY (Figs. 164, 165) within the tissues of the parent root and 

 in acropetal succession. The growing point of the new root is formed 

 from the innermost layer of the cortex in Pteridophytes and from 

 the pericycle in the Phanerogams ; a group of parenchymatous cells 

 commences to divide, the cells returning to the meristematic condition. 



