336 



MANUAL OF BOTANY 



vascular bundles are often spoken of as the stereome of the 

 plant. 



The cells of the parenchyma retain their protoplasm, and are 

 largely concerned in the metabolic processes of the plant. The 

 outer layers frequently contain chloroplastids, starch, &c. Gene- 

 rally in the axis of vascular plants the external layers of the 

 cortex differ to a variable extent from those nearer the centre, 

 and can be recognised as a well-marked region, often called 

 the hypoderma. The innermost layer, which abuts upon the 

 stele, or steles if there are more than one, constitutes the 

 endodermis. 



The hypoderma is often found to contain supporting tissue. 



Fie. 716. 



Fig. 717. 



—J 



Fig. 716. Diagram of section of monostelic stem of dicotyledonous plant. 



c. Cortex. &. Limit of stele, a. Vascular bundles. Fig. Ill, Diagram 



of seotion of moncstelie stem of monocotyledonous plant, a. Yascular 

 bundles, c. Cortex, b. Limit of stele. 



In many stems it is composed entirely of layers of sclerenchyma, 

 as in the rhizomes of many ferns and species of Equisetum. In 

 succulent petioles it is often composed of coUenchyma. 



In the hypoderma of roots the second layer of the cortex is 

 known as the exodermis (fig. 718). Its walls are usually thin 

 with a ladder-like thickening band extending round their radial 

 and upper and lower surfaces. In Monocotyledons it persists 

 throughout the life of the root ; in Dicotyledons it is soon shed 

 and replaced by a deeper corky formation. In some plants the 

 exodermis is several layers of cells in thickness. 



In foliage leaves the hypoderma sometimes shows a large 

 development of sclerenchyma, as in many of the Coniferte 



