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.! 
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 steve 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, 4 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 
be 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. 
