ORIGiN OF THE ROOT-CAP 



93 



of the young root expands laterally and then divides by a transverse 

 wall situated close to its anterior (or outer) face. The outer of the 

 two daughter-cells gives rise by further divisions and by appropriate 

 displacements of the resulting segments to a " hood-layer," which 

 reinforces the primary root-cap. The formation of this solitary hood- 

 layer appears to mark the termination of the development of the 

 root-cap at the expense of 

 the root proper ; the sub- 

 set [uent growth and regen- 

 eration of the root-cap 

 depend upon the activity 

 of its own innermost layer, 

 which thus constitutes the 

 calyptrogen. 



The fact that the root- 

 cap is made up, at any rate 

 during the early stages of 

 its development, of two 

 portions of totally distinct 

 origin, has subsequently 

 been confirmed for quite a 

 number of different families 

 by van Tieghem and Dou- 

 liot. In all such cases the 

 endodermis with or with- 

 out the assistance of the 

 adjacent cortical layers 



produces the outer apical envelope of the lateral root, termed by van 

 Tieghem the "digestive pouch" (poch.e or jtochr digestive); the name is 

 intended to suggest that the pouch secretes a digestive enzyme, which 

 dissolves the cortical tissue of the parent organ and thus opens a path 

 for the exit of the young rootlet. The inner apical envelope of the 

 rootlet, or " calyptra," is produced by the rootlet itself, which, it may be 

 remarked, always originates in the pericycle (pericambium). Typically, 

 therefore, the apex of the rootlet is, to begin with, covered by a double 

 envelope, consisting of a calyptra and a digestive pouch ; at a later 

 stage the pouch is cast off, and the rootlet is thenceforward covered 

 only by the calyptra. In the CRUCIFERAE, Ckassulaceae and 

 Ciienopodiaceae, in many Caryophyllaceae, among the Filicales and 

 in many other cases, there is no digestive pouch. In the Hydro- 

 charitaceae [Fig. 21 c], in Lemna, Pistia, Pontederia, and various other 

 water-plants, no calyptra is formed, and the entire root-cap therefore 

 corresponds to the pouch. 



Fio. 21. 



A. Transverse section through a root of Oryza sativa, 

 showing the origin of a lateral root; s, a, a, s, cells of the 

 innermost cortical layer, which enclose the primordium of 

 the rootlet. The cells a, a give rise to the primary root-cap. 

 B. Longitudinal section through a rootlet at a more advanced 

 stage of development (the surrounding cambial and cortical 

 cells are not shown); s, *, basal sheath ; a, a, primary root- 

 cap ; k, the primary hood-cell cut off from the terminal cell 

 of the median cell-row. C. Longitudinal section through the 

 primordium of a cauline adventitious root of Eloi/ea cana- 

 densis; h-h, cells of the stem-cortex, enveloping the root 

 proper, which by periclinal divisions give rise to the entire 

 root-cap. A and B after Nageli and Leitgeb. 



