DIV. I MORPHOLOGY 139 



The lateral roots break through the whole thickness of the cortex as 

 they emerge in the order of their development from the main root. 

 The ruptured cortex is frequently recognisable as a sort of collar 

 round the base of the lateral root. Other lateral roots may form 

 subsequently between those already developed and on older parts of 

 the root. 



The lateral roots always stand in VERTICAL SERIES on the parent 

 root ( 72 ). This arrangement is determined by their always arising 

 either opposite one of the longitudinally-running strands of xylem 

 (Fig. 164), or opposite the plate of conducting parenchyma which 

 separates a strand of xylem from one of phloem. The number 

 of vertical series of roots is thus either the same as the number of 

 strands of xylem, or twice this. In the former case the lateral 

 distance between any two adjacent roots is equal, while when the 

 roots arise right and left of a strand of xylem these two vertical 

 rows are approximated. 



The structure of the lateral roots corresponds with that of the 

 main root, and the xylem and phloem are continuous from the 

 one to the other. 



5. Roots borne on Shoots. Roots not only arise from other 

 roots but may be developed from the shoot, both from stems and 

 leaves. They are usually endogenous. In Ferns they arise from 

 meristematic tissue in the region of the growing point of the shoot. 



The place of origin of such adventitious roots is not fixed beforehand but may 

 be more or less definite. This is especially the case in marsh and water plants 

 where the roots arise from the lower nodes of the stem between, and alternating 

 with, the leaves ; they replace the primary root-system which has been lost when 

 the older part of the plant died off ( 73 ). They are especially numerous on the 

 under side of rhizomes (Fig. 138) and creeping shoots. A young shoot, or a 

 cutting planted in moist soil, quickly forms adventitious roots, and roots may 

 also arise in a similar manner from the bases of leaves, especially from Begonia 

 leaves when planted in soil ( 74 ). 



Dormant root-rudiments occur in the same manner as dormant buds of shoots. 

 Willow-twigs afford a special case of the presence of such dormant rudiments of 

 adventitious roots, the further development of which is easily induced by dark- 

 ness and moisture. 



6. Appearance of the Root- System. The lateral roots of 

 successively higher orders are as a rule thinner and grow less strongly 

 than their respective parent roots. The whole root -system is thus 

 typically RACEMOSE. The alternate branches are usually short arid 

 have a limited period of existence ; they may be termed ABSORBENT 



ROOTLETS. 



The root-system, like the shoot-system, further owes its general 

 appearance to the fact that the main and lateral branches take up 

 distinct positions in space relatively to one another ; this depends on 

 differences in their geotropism (cf. p. 339). 



