80 PEACTICAL BOTANY 



in the denser cell sap thus produced will pass on to the more 

 watery sap of adjacent cells. From these cells in turn por- 

 tions of sugar will pass on to other more distant cells. In a 

 similar way, when a potato tuber is planted and begins to 

 sprout, the sugar formed from the reserve starch in the potato 

 will pass into the more watery sap contained in the sprouts. 

 This sap is constantly losing sugar that is used as building 

 material for the young growing stems and leaves, and its 

 strength can be maintained only by the addition of new por- 

 tions of sugar coming from the tuber. 1 



73. Channels by which plant food is carried. 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 forming the sieve tubes, so called from the 

 perforated plates which are found at the ends or along the 

 sides of the nearly cylindrical cells of which the tubes are 

 built up. These sieve tubes in dicotyledons occupy a region 

 of the stem immediately outside of the cambium, as shown at 

 o in Fig. 29. 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 behavior 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. 

 Water meantime is freely carried upward through the sap wood. 

 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 gir- 

 dling process be more effective if a good deal of the sapwood 

 were removed from the ring as well as the bark ? Explain. 



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. 



