448 BOTANICAL GAZETTE [December 



Internal structure of vegetative organs 



ROOT SYSTEM 



The roots are neither contractile nor developed as storage 

 roots; they are simply nutritive and, although* the plant is an 

 annual, they increase quite considerably in thickness. This 

 increase takes place by means of the activity of the pericambium. 

 developing cork and a secondary cortex, beside by the development 

 of cambial strata inside the stele, in the manner typical of dicoty- 

 ledons. When fully matured the roots are very strong, owing to 

 the abundance of stereids outside the primary as well as outside 

 the secondary leptome, and in the secondary hadrome. The 

 primary structure may be studied from the apical portion of the 

 primary root of the seedling. The epidermis is hairy, but there is 

 no exodermis, and the cortex represents a compact parenchyma 

 of about ten layers without deposits of starch. The endodermis 

 is thin-walled, with Casparyan spots plainly visible. The peri- 

 cambium consists of a single continuous stratum, but is separated 

 from the stele proper by a layer of thin-walled parenchyma. Inside 

 this parenchyma are four strands of stereome with leptome on the 

 sides and on the inner face, beside four rays of hadrome, a small 

 pith occupying the center of the stele. With regard to the hadrome 

 the protohadrome vessels are annular and reticulated, mostly two 

 side by side, and much narrower than the inner, which vary from 

 reticulated to porous. At this stage there are no signs of cell 

 division in the pericambium, but narrow arches of cambial strata 

 appear between the leptome and hadrome, and none outside the 

 protohadrome vessels. The primary root is thus tetrarch, and 

 the increase in thickness commences by the development of cam- 

 bium between the leptome and hadrome. 



Examining this same root in its older portion near the base, 

 the following structure may be seen. Fig. 9 shows a diagram of the 

 stele and part of the peripheral tissues, of which epidermis, cortex . 

 and endodermis exhibit the same structure as described. Con- 

 cerning the pericambium there is now a slight indication of increase , 

 demonstrated by a tangential division (but only one) in each cell. 

 The stereome is now more thick-walled (fig. 10, St), and there are 

 many layers of cambium on the inner face of the leptome, also 





