TRANSLOCATION OF CARBOHYDRATES 277 



The connections with the spongy mesophyll and the very great 

 number of the fine bundle endings furnish an effective drainage 

 system for the rapid removal of the assimilates, which however 

 travel in the sieve-tubes and not in the sheaths themselves. 



If the path of the water supply to the palisade is considered 

 it is evident that after leaving the vessels of the finest veins the 

 water must pass into the bundle sheaths. Consequently the 

 sheath serves to distribute the water to the surrounding 

 mesophyll and it is clear that its structure well fits it for this 

 purpose. There is nothing improbable in the view that it also 

 carries out the converse process of receiving substances from 

 the assimilatory cells and passing them into the phloem for 

 removal from the leaf. In this connection it is interesting to 

 note that, in the English edition of Strasburger's Text Book oj 

 Botany, published in 1903, the passage of the sugars is described 

 as " . . . out of the mesophyll cells into the elongated cells of 

 the vascular bundle sheaths. The glucose and maltose are 

 transferred in these conducting sheaths through the leaf stalks 

 into the stem." 



But in the English edition of 1908, the addition "... and 

 from them into the phloem " is made after " bundle sheaths," 

 while the words "in these conducting sheaths" are omitted. 

 Thus, although still left somewhat indefinite, a step is made 

 towards restricting the choice of cells through which the sugars 

 may be translocated. 



The Starch Sheath. — Before leaving the bundle sheath and 

 its homologues it may be mentioned that Sachs considered that, 

 throughout the plant, the sugars travel more especially in the 

 innermost layer of the cortex. As pointed out above, the inner- 

 most layer of the cortex, considering the plant as a whole (the 

 "phleoterma" of Strasburger), forms in the leaf the single- 

 layered bundle sheath of the finer veins, but in the stronger 

 veins and in petioles and stems it is frequently differentiated 

 as a definite starch-containing layer. Heine pointed out the 

 improbability of this starch-containing tissue serving primarily 

 for the conduction of sugars longitudinally. As a rule its cells 

 are not markedly elongated and indeed are often almost cubical 

 (fig. 2). He was able to prove that interruptions made in the 

 sheath in various ways did not produce any noticeable changes in 

 the starch content, which, he considered, would not be the case 

 were the tissue a path for the longitudinal conduction of sugars. 



