TRANSLOCATION OF CARBOHYDRATES 273 



possibility of the sieve-tubes sharing in the removal of the 

 sugars. 



As stated above, there is no a priori reason against their 

 functioning in this way. But sieve-tubes. are not easy to examine 

 by Schimper's methods, especially when changes in their sugar 

 content are in question. He was able to follow the passage of 

 the sugars as far as the bundle sheaths and then drew his 

 conclusions. The fact that sugars actually do pass in large 

 quantities into the sieve-tubes as well could only be shown 

 clearly when more suitable methods became available. How- 

 ever Schimper recognised that there were two main possibilities, 

 either that the sugar left the leaf only in the tissue outside the 

 vascular bundle proper or that the latter was necessary for 

 the normal process of depletion. That he decided in favour 

 of the first alternative was perhaps mainly due to the limitations 

 of the methods he employed. 



Turning to the anatomy of the bundle sheath, it is interesting 

 to note that Schubert found this present in the leaves of all 

 the numerous plants he examined, except members of the 

 Crassulaceae. The metabolism of these plants is of a sluggish 

 nature and it might be urged that they have no need for any 

 special contrivance for the removal of large quantities of 

 assimilates. In the other plants Schubert found that the fine 

 veins in the leaves, consisting of one or two tracheids or vessels, 

 with a sieve-tube or transition-cell, are surrounded by a layer of 

 cells markedly distinguished from the neighbouring parenchyma. 

 Their inner walls are in close contact with the elements of the 

 bundle ; their lateral walls are placed radially and also show 

 no intercellular spaces. The cells are often (but not invariably) 

 longer than broad, have thin walls and frequently few or no 

 chlorophyll granules, though there are exceptions to the last 

 characteristic. Seen in transverse section they form a very 

 distinct ring on account of their close approximation and 

 regularity of arrangement. (Figs, i, 12.) 



The larger vascular bundles also are surrounded by paren- 

 chymatous sheaths, in which, however, more than one layer is 

 formed by the tangential division of the cells and the volume 

 of the tissue is thus increased. This increase in the number of 

 layers of cells is chiefly noticeable on the upper and lower sides 

 of the main veins, where it iorms the tissue known as "nerve 

 parenchyma." The structure of the cells of this tissue closely 



