STRUCTURE OF THE SHOOT 69 



sections through the internodes of several dicotyledonous 

 stems, shows that the vascular bundles are arranged in the 

 form of a ring. Fig. 32 shows one of the vascular bundles 

 of the Buttercup, highly magnified. Note the three dis- 

 tinct groups of tissue of which it is composed. On the 

 outside is a group of delicate cells, the bast or phloem (b) ; 

 the larger elements sieve tubes (s) are accompanied by 

 very small ones, called companion cells (c) ; both are hlled 

 with organic materials. On the inner side of the bast is 

 a band of narrow, flattened cells the cambium (ca). In 

 that portion adjoining the pith is the wood or xylem (w), 

 composed of wide, tubular, thick-walled vessels, among 

 which are narrower, thick-walled woody fibres. The veins, 

 as in the root, are arranged in the form of a network and do 

 not continue the parallel course which a section through 

 the internode might suggest. Numerous examples of this 

 may be obtained from waste heaps where shoots such as 

 old cabbage-stalks are undergoing decay. 



With a little trouble, the above details may be made out 

 by carefully dissecting the stem of a Deadnettle which has 

 previously been boiled in water for about twenty minutes. 

 Tougher stems may be boiled in water to which a little 

 caustic potash has been added. This softens the cortical 

 tissues so that they may be brushed away from the veins, 

 as was done in preparing the skeleton of the Box leaf. If 

 a piece of stem including two or three pairs of leaves be 

 selected, it will be seen that the veins, passing from the 

 leaves down the leaf-stalks, enter the stem, branch at the 

 nodes, and join on to neighbouring veins. This will be clear 

 from a study of Fig. 33. By means of these veins direct 

 communication is set up between roots, stem, and leaves. 



Fig. 34 shows the arrangement of these tissues in a trans- 

 verse section of the Deadnettle stem. At the corners are 

 the large fused bundles from the leaves (v.b), while at the 

 sides are the small bundles. In this stem the innermost 



