122 
OTIS F. CURTIS 
The experiments with ringing dormant stems indicate that the growth 
above a ring practically ceases when the carbohydrates in this region have 
disappeared. Even the starch of the xylem itself is not digested and re- 
moved upward unless a path through the phloem is open. If that part 
above the ring is so short that the bud merely elongates but does not open 
its leaves, growth ceases completely, and later the enlarged bud or partially 
opened leaves wither and die. If sufficient food was above the ring to 
enable one or two leaves to open, growth often slowly recommences, prob- 
ably because of the food made available by the new leaves. 
The experiments with double ringing show even more clearly that the 
xylem does not serve as a tissue for longitudinal translocation of the car- 
bohydrates stored in it, for between rings the content both of starch and of 
soluble sugars is much higher than either above or below the part thus iso- 
lated. It would seem that the removal of carbohydrates from the xylem 
occurs only radially through the medullary rays and that longitudinal 
transfer in either direction is through the phloem only. In a ring i cm. 
wide starch was still present when all the starch had disappeared imme- 
diately above the ring. Later, however, the upper part had become de- 
pleted while the lower part was still full. Diffusion may have accounted 
largely for the movement through this short distance. Observation on 
this point has not been made, but it seems probable that much of the food 
from between rings will be eventually used up in cambial growth. 
As was previously stated, Atkins (1916), finding appreciable quantities 
of sugars in the sap centrifuged from the stems of various woody plants, 
concluded that these must move upward with the "transpiration stream" 
and that one of the principal functions of the xylem is to transfer these 
sugars. The data here recorded, however, show that no appreciable 
amounts of sugar are transferred vertically through the xylem. In the 
experiments in which the leaves were removed above a ring, it seems possible 
that the lack of rapid transpiration might partly account for the failure of 
the xylem to transfer food; however, in the experiments in which dormant 
wood was ringed some distance from the tip plenty of leaves were produced 
above the ring to carry on rapid transpiration, yet this seemed to have 
no effect in causing the removal of either starch or soluble sugar below the 
ring, or from between rings in those stems that were double-ringed. In the 
the maple reported in tables 12-15, and in the beech reported in tables 16-18, 
there was a very large leaf surface that must have required quantities of 
water, but even the soluble sugars were not removed from between rings. 
It will be necessary to make further tests on centrifuging the xylem before 
definite conclusions can be safely drawn, but the little that has been done 
at least suggests that even the sugar in the vessels does not normally move 
with the water in transpiration. We know that when a tree, such as the 
maple, is tapped, sugar solution will flow through the vessels. This is not 
proof, however, that in an uninjured tree there is any flow of solution through 
