FOODS IN PLANTS 43 



and the supply of materials for growth is maintained by the 

 addition of new portions of sugar coming from the tuber. 1 



42. Channels through which food is transported. Many 

 kinds of living tissue serve as channels for the conveyance 

 of food from one part of the plant body to another. The 

 main route for the transportation of food in flowering plants 

 is through special tubular cells which form the sieve tubes, so 

 called from the perforated plates found at the ends or along 

 the sides of the nearly cylindrical cells of which the tubes 

 are built up. In dicotyledons these sieve tubes occupy a 

 region of the stem immediately outside of the cambium, as 

 shown at o in figure 42, A and at si in figure 43. The fact that 

 most of the plant food prepared in the leaves is carried down 

 through the sieve layer of the bark is well shown by the be- 

 havior of a willow cutting from which a ring of bark has been 

 removed. If the cutting is stood with its lower end in water 

 but with the girdled part out of water, enough constructive 

 material will pass down through the sieve layer to send out 

 roots from the upper edge of the ring, but few or none will 

 appear at its lower edge. In the meantime water is freely 

 carried upward through the sapwood. In early times the 

 process of clearing woodlands for farming purposes was 

 made less laborious by girdling the trees, which soon died 

 and at length fell and were burned. Would the girdling 

 process be more effective if a good deal of the sapwood, as 

 well as the bark, were removed from the ring ? In woody 

 dicotyledonous stems there are radiating lines of cells (med- 

 ullary rays) running outward from the center toward the cir- 

 cumference of the stem. Food is stored in these rays, and 

 they are also lines of conduction of foods. 



43. Food storage. In the trunks of trees stored food may 

 l>e present in various forms, as starch, sugar, oil, and proteins. 

 In the autumn many kinds of sapwood turn deep blue or black 



1 It is not possible here to go into details concerning the transportation 

 of other kinds of plant food than starch and the sugars. That of proteins is 

 especially difficult to trace. 



