104 



TRANSPORT OF WATER AND SOIL SOLUTES 



system comparable in this respect to the anastomosing veins and 

 arteries of the animal body. 



It will be seen that if the leaves on one side of a tree are made 

 to transpire faster than those on the other because of greater 



exposure to the sun or 

 drying wind from that 

 quarter they can draw 

 upon the water in the 

 whole circle of bundles 

 in the trunk; or when 

 one side of a stem is 

 severely injured so as 

 to interrupt the flow of 

 water on that side, the 

 leaves above the injured 

 part can draw their 

 water from the opposite 

 side, which in turn can 

 reemburse itself from 

 the injured side below 

 the wound. 



The vascular bundles 

 that descend from a 

 leaf into the stem con- 

 stitute a leaf trace. 

 The number of bun- 

 dles in each leaf trace 

 varies commonly from 

 one to three, but there 



may be more. The rule is that the whole ring of vascular 

 bundles in Dicotyledons is composed of leaf traces, but, as was 

 stated above, instances occur where bundles do not extend into 

 the leaves, and these anomalously are in the pith or even in the 

 cortex, and since they do not run out of the stem they are called 

 cauline bundles, while the others that run into the leaves are 

 called common bundles. 



FIG. 49. A, diagram of course of vascular bundles 

 in a palm stem; mi, 2in, 3>n, bundles from the median 

 portions of the leaves. B, diagram of vascular bundles 

 in external view and in cross section of the stem of 

 Cerastium. The leaves are shown cut off at i, 2, 2' 

 and 3. (After Vines.) 



