THE ANGIOSPERMAE : ROOTS 805 



The lateral branches of the root originate endogenously from the margin 

 of the stele, and this again is a rule with very few exceptions, though it is 

 well to realize that exceptions exist even to this most characteristic feature 

 of roots. 



Biologically the endogeny of root branches may be interpreted as pro- 

 tective. There being no leaves to form a protective bud, the main root has 

 to rely upon the covering afforded by the root cap, while the external tissues 

 of the parent root serve the branches in the same way until they have formed 

 their own root caps. Moreover this method of origin delays their appearance 

 at the surface, and they may be several centimetres behind the apex of the 

 main root before they break out. Thus the forward growth of the main 

 apex is not hindered by horizontal outgrowths, nor is the physiologically 

 important zone of the root hairs interrupted. Exceptions to this rule may be 

 found among water plants, where the above considerations do not apply. 

 For example, in Eichorma, the Water Hyacinth, the branch rudiments are 

 formed in the apical meristem and emerge almost immediately from the 

 parent root. 



The outer layers of the root become differentiated and specialized at a 

 very early stage, while the inner layers retain their embryonic character much 

 later, especially the pericycle. No doubt this is associated with the later as 

 well as with the endogenous origin of the branches. 



In all types of root having more than two xylem groups the laterals 

 originate opposite the protoxylems. There are thus the same number of 

 longitudinal rows of branches as there are xylem groups in the main root. 

 Diarch roots are a notable exception to this rule. Their branches form four 

 rows opposite each of the spaces between the xylem and phloem groups. 

 A special feature of diarch root branches is that in Vascular Cryptogams 

 the xylem plate is typically horizontal, whereas in Angiosperms it is always 

 vertical. The Grasses and other Monocotyledons having a large number of 

 root xylems do not, however, produce branches opposite all of them. 



When a root branch is initiated the pericycle cells divide both radially 

 and tangentially, a disc-shaped body of cells being formed, which increases 

 into a rounded boss and pushes the endodermis outwards in front of it. The 

 endodermal cells also begin to divide and they form a cap over the developing 

 branch apex. This portion of the endodermis breaks away from the rest 

 and is carried outwards through the cortex by the developing branch. The 

 cortical cells disintegrate in front of it, and van Tieghem called the sac thus 

 formed the " digestive pouch," attributing it to enzyme action. This is 

 doubtful, the action being probably due purely to mechanical pressure. 

 jVIeanwhile the branch apex is differentiated into histogen zones (Fig. 799). 

 It develops its own root cap, and vascular elements differentiate acro- 

 petally in its plerome, beginning at a point in contact with the corresponding 

 tissues of the parent root. On finally emerging through the ruptured 

 exodermis, the endodermal cap is sloughed off and the young apex remains 

 covered by its own cap, except, as mentioned above, in some Monocotyledons, 

 where the branch roots form no calyptra of their own. 



