ix THE HISTOLOGY OF THE SHOOT AND ROOT 113 



the stem, including the pith, the bundles, and pericycle. This 

 layer receives the name oi plerome. 



Formation of Leaves. Leaves are formed from the der- 

 matogen and periblem. The dermatogen grows out and the 

 periblem follows. From the dermatogen the epidermis only is 

 formed, the mesophyll and vascular bundles being formed from 

 the periblem. 



Formation of Branches. When a branch arises in the 

 axil of a leaf, it is formed from the dermatogen and periblem, 

 the plerome taking no part in it. Thus a branch is produced 

 from the outer tissues of the stem, />., from the cortex and 

 epidermis, and it is said to be formed exogenously. 



The Growing Point of the Root. All the new tissues of 

 the root are produced from its apex. Though there are no 



FIG. 140. A transverse section of the 

 stem of Ivy ; showing origin of aerial 

 root. (X 8.) 



FIG. 141. Root cap of Barley. 

 (Magnified.) 



leaves to be developed, there is a root-cap to form. If the young 

 root of a Bean plant is held up to the light, two parts can be 

 distinguished, a lighter outer portion and a darker inner portion. 

 The outer portion is the root-cap, and the inner dark zone the 

 growing point, which is protected by the root-cap. The growing 

 point consists of three layers, as in the shoot. They are : 



(i.) The plerome, which forms the vascular cylinder. 



(2.) The periblem, which forms the cortex, 



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