MORPHOLOGY OF HIGHER PLANTS. 313 



phloem and xylem, as separate strands alternating with each other 

 and not being located, as in stems, in the same radii, has given 

 rise to several adverse views. Some authors have considered 

 the root-stele as one single mestome-strand (or fibrovasculaf 

 strand), while others, especially of recent date, consider it to be 

 composed of several MESTOME STRANDS. 



The xylem or hadrome contains tracheae or vessels, the periph- 

 eral being spiral and narrower than the inner, which are scalari- 

 form or reticulate. The tissue in the center of the stele in mono- 

 cotyledons is not uncommonly made up of parenchyma cells, and 



01 Liacueic; r, lepiorne. composed 01 sieve ct 

 forming a triarch radial fibrovascular bundle. 



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 Gymnosperms and Dicotyledons), the increase 

 depends upon the activity in the pericambium, some of the cells 

 becoming meristematic. These meristematic cells are known as 

 phellogen, developing cork outwardly and secondary cortex in- 

 wardly. The meristem of the stele or cambium also becomes very 

 active and develops on the inner face of the phloem and extends 



