200 



BOTANY AND PHARMACOGNOSY. 



words, the original radial structure of the stele changes to the 

 collateral type (Fig. 112). Owing to this increase within the 

 stele, the peripheral tissues from the endodermis to the epidermis, 

 naturally become broken and are subsequently thrown off, but are 

 replaced by the pericambial cork and secondary cortex. The 

 older roots, then of Gymnosperms and Dicotyledons thus resemble 

 the structure of stems, except that no pith exists in these roots, 

 at least not usually. Some differences are, however, quite notice- 



Fin. no. Primary structure in the root. Transverse section of root of pea (Pisum) 

 about 40 mm. from the root-cap: H, epidermal cells, some of which are developed into 

 root hairs; C, primary cortex; EN, endodermis; PC, pericambium; X, ladrome, composed 

 of trachea; P, leptome, composed of sieve cells, the hadrome (vessels) and leptome (sieve) 

 forming a triarch radial fibrovascular bvmdle. 



able in some instances as in the thick roots of Beta, Radish, etc., 

 where the wood parenchyma is usually abundant, thin-walled and 

 not lignified, the annual rings also being mostly indistinct. 



The characteristics distinguishing the primary and secondary 

 structures of dicotyledonous roots may be summarized as follows : 



Primary structure : Epidermis and root hairs. Hypoder- 

 mis. Primary cortex consisting of parenchyma. Endodermis, 

 pericambium, hadrome arranged in radial rays which alternate 



