ANGIOSPERMS. 



517 



Figs. 373 and 374, that it is simply a luxuriant growth of the dermatogen. This 

 peripheral layer of tissue, which elsewhere remains simple, and passes over into 

 permanent tissue in forming the epidermis, increases in thickness, on the contrary, 

 where it covers the puncium vegetaiionis of the root, and undergoes repeated 

 tangential divisions (parallel to the surface). Of the two layers which are succes- 

 sively formed on each of these occasions, the outer becomes a layer of the 

 root-cap (Fig. 373 wh, and Fig. 374, 2); the inner remains as dermatogen and 

 again undergoes the same process. This dermatogen which covers the vegetative 

 cone of the root behaves therefore like a layer of phellogen, with this difference, 

 that the cells produced from cork-cambium become at once permanent cells, while 

 those of the root-cap remain still capable of division; so that each layer split off 

 as it were from the dermatogen forms a cap consisting of several layers of cells ; 

 its growth being most active in the centre, and diminishing towards the periphery. 

 The splitting of the dermatogen into two lamellae usually progresses from the 



V\C,. -573. — Diagrammatic rcpresentatif>n of tlie 

 formation of the primary root in Monototylcilons 

 and its connection with the stem (after Hansteiii) ; 

 V pro-embryo, h liypophysis, iv iu line of separation of 

 the root and stem, ivh layer of the root-cap, d derma- 

 togen, pb pcriblem, // pleronie. 



Fig. 374. — Diagi-ammatic representation of the 

 formation of the embryo of Dicotyledons (after 

 Hanstein) ; i, 2, the first layer Of the root-cap,/ peri- 

 blem, d dermatogen, pi plerome. 



apex towards the periphery of the apex of the root ; in the secondary roots of 

 Trapa, Hanstein and Reinke state that the reverse is the case. 



Lateral roots not unfrequently arise in the embryo even before the ripening of 

 the seed, in addition to the primary root which we have hitherto alone considered ; 

 as, for instance, in many Grasses and some Dicotyledons {e.g. Impatiens, according 

 to Hanstein and Reinke, Cucurbita from my own observations). In Trapa natans 

 the primary root soon becomes abortive, laferal roots arising at an early period from 

 the hypocotyledonary portion of the axis. 



Hanstein and Reinke state that the lateral roots of Angiosperms have their 

 origin in the pericambium, in Nageli's sense of the term\ Their development was 

 found in several plants to harmonise with this. In Trapa naians, for example, it is 

 as follows : — A group of cells of the mantle of pericambium which consists of only 

 one layer divides radially; the newly formed cells elongate in the same direction, 

 and then divide tangentially ; the outer of the two layers produces the dermatogen, 



* Compare what was said on Fig. 1 15, p. 145. 



