300 RELATION TO ENVIRONMENT. 



(fig. 417). Surrounding the plerome and filling the space between it and 

 the dermatogen is the third formative tissue called the periblem, which later 

 forms the cortex (bark or rind), and consists of parenchyma, collenchyma, 

 sclerenchyma, or cork, etc., as the case may be. It should be understood 

 that all these different forms and kinds of cells have been derived from 

 meristem by gradual change. In the mature stems, therefore, there are 

 three distinct regions, the central cylinder or stele, the cortex, and the 

 epidermis. 



710. Central cylinder or stele. As the central cylinder is organized from 

 the plerome it becomes differentiated into the vascular bundles, the pith, 

 the pith rays (medullary rays) which radiate from the pith in the center 

 between the bundles out to the cortex, and the pericycle, a layer of cells 

 lying between the central cylinder and the cortex. The bundles then are 

 farther organized into the xylem and phloem portions with their different 

 elements, and the fascicular cambium (meristem) separating the xylem 

 and phloem, as described in Chapter V. Such a bundle, where the xylem 

 and phloem portions are separated by the cambium is called an open bun- 



Fig. 418. 



Concentric bundle from stem of Polypodium yulgare. Xylem in the center, 

 surrounded by phloem, and this by the endodermis. (From the author's Biology 

 of Ferns.) 



die (as in fig. 58). Where the phloem and xylem lie side by side in the same 

 radius the bundle is a collateral one. Dicotyledons and conifers are char- 

 acterized by open collateral bundles. This is why trees and many other 



