stem Structure 



cambium, which by its division adds to both the 

 wood and the soft bast and so thickens the stem. 

 This part of the stem is really a series of cords lying 

 side by side, each consisting of wood inside, cam- 

 bium, and soft bast. In other plants there is no ring : 

 instead, the bundles of wood and soft bast tissue are 

 scattered about, forming long cords through the stem 

 tissues, and no cambium is present. In any case, one 

 end of the cords or their branches lies in the leaves 

 where the veins are made by them, and the other in 

 the roots, where they reach the uttermost rootlets. 



The wood consists of fibres for strengthening, and 

 long tubular vessels along which water or earth- 

 salts are pushed and drawn up from the root to the 

 leaves. All water passes up the wood. 



The soft bast consists mainly of pecuUar tubes 

 along which the food made in the leaves passes to 

 any part at which there is a demand for it, arising 

 either from its being used to form new tissues, or in 

 breathing, or through its storage. 



There is therefore an upward passage of water 

 containing earth-salts, and a downward passage of 

 water containing manufactured food, and each has 

 its own proper channels. The channel for the water 

 and earth-salts Ues toward the inner part of the stem 

 and the upper side of the leaves ; the channel for the 

 food on the outer part of the stem (just on the inner 

 side of the bark) and along the lower side of the leaf- 

 veins. Have you noticed aphides feed on the lower 



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