46 TRANSLOCATION IN PLANTS 



but that made during the current year also fails to be transported through 

 the wood. This was demonstrated by experiments in which the continued 

 growth of the apex of ringed defoliated shoots was found to be greatly 

 reduced when compared with that of similar defoliated shoots not ringed. 

 The amount of growth of ringed leafy shoots is proportional to the leaf area 

 above the ring and, when several leaves are present, is approximately the 

 same as that of normal shoots for the first few days. These results indicate 

 that food necessary for growth moves up through the phloem and not the 

 xylem, and that this food is probably largely carbohydrate because it is 

 produced in leaves. That ringing interferes with upward transport of foods 

 was indicated not only by diminished growth of ringed defoliated stems but 

 by the fact that they also had a low sugar content and low osmotic concen- 

 tration when compared with similar shoots not ringed. The occasional 

 withering of ringed defoliated shoots is demonstrated to be due, not to any 

 injury to the xylem, but to the low osmotic concentration making the parts 

 unable to compete with other tissues for water. That the failure to carry 

 sugar through the wood is not due to the lack of a transpiration stream is 

 demonstrated by experiments showing no movement into defoliated stems 

 if they are isolated from leaves below by a ring, but ready movement if not 

 so isolated. Leaves were present above in both cases to insure a flow of 

 water through the stem. Other experiments on the ringing of branches 

 with leaves lacking chlorophyll as well as experiments with cotton where 

 the xylem is separated from the wood demonstrate that upward transport 

 of carbohydrates is limited to the phloem. 



8. Experiments of several sorts are cited which demonstrate that food 

 moving to fruits is carried exclusively through the phloem. This is true 

 independently of whether the food is coming from the leaves situated above 

 the fruit or below it. If the phloem is intact, food may move distances up 

 to three meters to fruits on defoliated branches. 



9. In order to determine more certainly whether xylem or phloem is 

 chiefly concerned in upward transfer, and in order to eliminate the criticism 

 that ringing stops transfer because it results in injury to the xylem, experi- 

 ments were carried out in which the xylem was removed and the phloem 

 left intact. Such experiments were performed with defoliated shoots and 

 the amounts of growth above the operated regions compared. In experi- 

 ments of this sort with many kinds of plants it was found that cutting the 

 phloem prevented food transfer, whereas cutting the xylem allowed for 

 approximately normal transfer. That the cambium is not necessary for 

 transport was indicated by experiments in which both xylem and cambium 

 were removed. 



