OTHER STEMS 



2S7 



Figure 286. — Whip Grafting. 



which twines around some support and thus is able to gel 

 sunlight for its many leaves. Twining plants of the same 

 kind always curve in the same direction. In twining 



around any object they 



touch, climbing plants 



are responding to thig- 



motropism. 



The wild grapevine is 



a plant which climbs to 



the top of trees by 



means of a long, leaf- 

 less stem. Such plants, 



common in the forests 



of tropical countries, are 



called lianas. 



Woody stems have a 



structure which differs 



from that of the soft bean stem. On the outside is the 



brown bark in which are lenticeh, holes which allow air t<> 



enter. Under this is a layer of green bark, the out» *r 



edge of the phloem of the vascular bundles. Between the 



phloem and the xvlein of 

 each vascular bundle is a 

 region of rapidly dividing 

 cells, which is called the 

 CCDnhilUH, When the \ as- 

 cular bundles are crowded 

 close together the cam- 

 bium of adjoining bundles 

 touches, thus forming a 



ring around the tree (Figure 284). The outer edge of this 



cambium layer is always turning to phloem, and the 



inner to xyleni. 



A woody twig like the horse-chestnut (Figure 290) has 



Figure 287.- — Budding. 



