THE STEM 



245 



which we have seen as the covering of leaves. Tho area sur- 

 rounded by the epidermis (C. T.) consists of a cellular tissue with 

 several majrked groups of a separate tissue, known as vascular 

 bundles (F.). These bundles are variously grouped in the two 



-C. T. 



V. 



Fig. 225. — Diagrams of young stems: I. of a dicotyledonous, II. of a monocoty- 

 ledonous tree. E. Epidermis, C T. Cellular tissue. V. Fibro-vascular bundles. 

 The latter consist of an inner woody part (TF) and an outer part called Bast (B). 

 In the dicotyledonous tree there is a Cambium layer (C) between the two parts. 

 The Cellular tissue {C. T.) will in older trees be differentiated into Pith (M), Medul- 

 lary Kays {M. /?.), and Bark [R). 



chief classes of flowering plants, i. e., the dicotyledons and the 

 monocotyledons. 



{ci) The arrangement of the vascular bundles in dicotyledons 

 can be nicely seen in sections of the stem of Aristolochia: they 

 are located radially in a circle. The cellular tissue is thereby 

 divided into two distinct parts: the pith (Ji), which lies inside, 

 and the rind or cortex (E), which lies outside the ring of vascular 

 bundles. Those parts of the cellular tissue, which separate the 

 several bundles and connect pith with cortex, are called medullary 

 rays {M. R.). 



{b) On sections of monocotyledonous stems (Palm tree, Maize) 

 we can see that the vascular bundles are scattered irregularly 

 in the cellular tissue. Hence there is no distinct division of the 

 cellular tissue into pith, cortex and medullary rays. 



