"J 6 Plants and their Ways in South Africa 



and develop into the three parts of the bundle : its outermost 

 cells form the first phloem (protophloem), the innermost the 

 protoxylem, while between the two regions the procambium 

 remains active and forms the cambium of the bundle. 



{c) The cells of the ground tissue divide in all directions 

 and become marked out into the medulla, the medullary rays, 

 the pericycle, and the cortex. 



Leaf Formation. — The dermatogen and underlying 

 ground tissue of the stem give rise to the leaf. The cells at a 

 node divide actively and form the leaf protuberance. The 

 vascular bundle supplying the leaf, begins forming at the node ; 

 the cells extending up into the leaf and down into the stem 

 gradually elongate until a continuous bundle is formed con- 

 necting the leaf with the food supply. In dicotyledons the 

 number of leaf bundles or leaf traces is relatively few ; they 

 run down the stem parallel to one another through one or more 

 internodes and fuse at the nodes. The bundles are common, 

 that is common to leaf and stem. The course they take as 

 they pass down the stem forms the ring which is seen in a 

 cross section of an internode. 



B. Monocotyledonous Stem. — A monocoytledonous 

 stem is distinguished from that of a Dicotyledon by the fact 

 that in the former the bundles are closed ; that is all the 

 procambium passes over into phloem and xylem so that further 

 increase is prevented. 



The bundles appear to be scattered irregularly ; as a matter 

 of fact they are arranged in more or less concentric circles. 

 The bundles are common, but in monocotyledons, which as a 

 rule have broad leaf bases, the leaf traces are more numerous, 

 and instead of running down the stem parallel to the surface 

 as in dicotyledons, they curve in the stem ; the midribs making 

 the largest curve extend nearly to the centre of the stem, 

 while the lateral veins of the leaf do not penetrate so deeply. 



In many monocotyledons it is difficult to distinguish cortex 

 from stele as the bundles extend out from the centre close to 

 the circumference as in Zea mays ; in others, Omithogalum for 

 example, a well-developed ring of fibres in the pericycle makes 

 the distinction evident. In Zea where this ring is wanting, 



