59* 



PHANEROGAMS. 



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 lamella? usually progresses from the 

 centre 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. Impa/t'ens, according 

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

 the primary root soon becomes abortive, lateral 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 



d P^ ?f 



Fig. 404. — Diagrammatic representation of the 

 formation of the primary root in Monocotyledons and 

 its connection with the stem (after Hanstein) ; v sus- 

 pensor, h hypophysis, iv w line of separation of the root 

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

 pb periblem, pi plerome. 



Fig. 405. — Diagrammatic representation of the 

 same in the case of Dicotyledons (after Hanstein); 

 I, 2, the first layers of the root-cap, p periblem, a? derma- 

 togen,// plerome. 



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

 as follows : — A group of cells of the 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, the inner 

 the body of the root. The dermatogen, pushed outwards by the development of the 

 body of the root, produces the root-cap in the way already mentioned ; the tissue of 

 the body of the root itself which is covered by it becomes differentiated into plerome 

 and periblem. The same process takes place in PisHa, and probably also in 

 Grasses. Hanstein and Reinke do not find 'anywhere an apical cell which ori- 

 ginates the growth, as in Cryptogams, but a group of cells which obey a common 

 law of growth.' 



The variation in the size of the embryo in the ripe seed of Angiosperms has 



^ [See the account of the apical growths of roots and of the development of lateral roots which 

 is given in the Appendix.] 



^ Compare what was said on Fig. 125, p. 167. 



