STORAGE AND TRANSLOCATION 285 



in some plants it seems to have no effect. In squash, for example, 

 removing a ring of tissue down as far as the wood of the bundles 

 does not have the effect which one might expect. The cucurbits, 

 however, have the bicolateral bundles mentioned above, with 

 phloem on both sides of the xylem, so that only a part of the phloem 

 is interrupted by the experiment. Likewise girdling is of little 

 effect in monocots, which have the bundles scattered throughout 

 the stem. 



Girdling is much practiced in certain regions where it is desired 

 to produce unusually large fruit. A ring or girdle is made some 

 distance from the fruit, permitting the fruiting branch to have 

 several leaves. Then all but one or two fruits are removed from 

 the branch, with the result that all the food produced in the 

 branch goes into these particular fruits. In conducting such 

 experiments, however, one must be careful not to girdle in such a 

 way that the plant will be permanently injured. 



The facts that (a) translocation from leaves on one side of a 

 plant tends to result in storage on that same side in many species 

 of plants (Caldwell, 1928), and that (b) chilling a petiole slows up 

 translocation more than it does water transportation (Curtis, 

 1929) point to the conclusions that the region of translocation 

 (a) is not in the center of the stem and (b) that it is composed of. 



living cells. 



All of these evidences point to the phloem, therefore, as the 

 region of translocation. Each type of evidence is not conclusive 

 in itself, but the entire array of facts points unmistakably to the 

 phloem as the responsible region. 



Structure of Phloem and Its Method of Operation.— The 

 phloem is found to consist of two kinds of cells,— the sieve tubes 

 and companion cells. The former are elongated cells with per- 

 forated ends which permit the passage of colloidal materials such 

 as the proteins. The companion cells (Fig. 17), which are smaller 

 in cross section but of the same length, are found in close prox- 

 imity to the sieve tubes. The sieve tubes lose their own nuclei 

 early in their development, but the protoplasm, which lines their 

 cell walls, remains contiguous with that of the adjacent companion 

 cells and is " under the domination" or " control" of the com- 

 panion cell nuclei. 



Just how the phloem carries on its work and the role played by 

 the various elements, are questions still unsettled. The cytc- 



