70 



The elements and functions of tlie Bast and Wood 

 are the same as those already described unde 

 Stem Bundles. 



OEIGIN OF LATERAL ROOTS. 



One cell of the Endodermis outside each protoxylem 

 group grows larger than its neighbours. It divides 

 up and forms a little cone of tissue. This '' cone " 

 increases in length and makes its way through the 

 tissue of the cortex by absorbing the cells which are 

 in its way. When it reaches the pilifery this too is 

 absorbed, and finally its tip appears outside the 

 mother root. It, even in this young stage, is found to 

 be protected by a cap. It always exhibits a structure 

 identical with its parent, and is said to be endogenous 

 in its origin, because it arises in the deeply-seated 

 endodermis. Lateral roots of the higher plants arise 

 in the pericycle. The innermost layer of the cortex 

 sometimes keeps up with the growth while it is still 

 within the parent root ; this cortical layer is then 

 described as the Digestive Sac, because it assimilates 

 the tissues of the mother root and conveys the proto- 

 plasm so obtained to the growing lateral root. The 

 Digestive Sac is itself absorbed just before the 

 lateral root emerges. In the higher plants the endo- 

 dermis forms the Digestive Sac. 



APICAL GROWTH. 



Roots, stems and leaves grow by means of one cell. 

 This cell presents the appearance of a solid triangle. 



