MORPHOLOGY OF HIGHER PLANTS. 199 



The hadrome contains tracheae or vessels, the peripheral being 

 spiral and narrower than the inner, which are scalariform or 

 reticulate. The tissue in tlie center of the stele in monocoty- 

 ledons is not uncommonly made up of parenchyma cells, and 

 corresponds exactly with the pith of the stem. In roots it is often 

 called CONJUNCTIVE tissue and the cells may contain starch and 

 crystals of calcium oxalate. 



Secondary Structure. In roots that are able to increase in 

 thickness (as in Gynmosperms and Dicotyledons), the increase 

 depends upon the activity of the pericambium, which develops 

 cork outwardly and secondary cortex inwardly, and on the de- 

 velopment of a cambium. The latter develops on the inner face 



RB 



Fig. 109. A transverse section through the root of a germinating pea-plant (Pisum) 

 about 40 mm. from the tip, showing the origin of a root-branch (RB); E, epidermis; C, pri- 

 mary cortex; X, hadrome (vessels); P, leptome (sieve); EN.'endodermis. 



of the leptome and extends from there to the outside of the 

 peripheral vessels of the hadrome (Fig. iii), thus a continuous 

 cambial zone gradually arises. From this zone secondary 

 tracheae or vessels become developed on the inner face of the 

 primary leptome, while secondary leptome becomes differentiated 

 outside the primary rays of hadrome ; or only parenchyma develops 

 outside the primary hadrome, resulting in the formation of 

 secondary parenchyma-rays (or medullary rays). In other 



