356 PHYSIOLOGY OF NUTRITION. 



starch undoubtedly takes place chiefly in the cortex and pith. 

 The starch sheath only contains much starch when the elements 

 of the bast fibre layer of the bundles are not yet fully developed. 

 With advancing development of these the starch gradually dis- 

 appears from the cells of the starch sheath, being employed in 

 building up the thick- walled bast elements. 1 



The fact has already been mentioned that many plants have the 

 power of reconverting transitorily into starch, along the channels 

 of conduction (the leaf -nerves), the carbohydrates which have 

 migrated from the mesophyll of the leaves. Other plants can 

 effect this only to a small extent, and hence we find their leaf- 

 nerves filled not with starch but with glucose. We prepare 

 transverse sections from the lower part of the lamina and the upper 

 part of the leaf-stalk of a mature turnip leaf. The starch formed in 

 the blade by assimilation travels under normal conditions through 

 the nerves and the leaf -stalk into the root, rendering possible its 

 development. By micro-chemical tests, however, we find only very 

 small quantities of starch in the parenchyma which surrounds 

 the vascular bundles of the nerves and of the leaf -stalk, whereas 

 we find very large quantities of glucose, and we may therefore 

 designate the tissue conducting the carbohydrate a conducting 

 sheath, and in particular a sugar sheath. 2 



1 See H. Heine, Berichte tier Deutschen botan. Gesellscliaft, Bd. 3, Heft 5. 



2 See H. de Vries, Landwirthschl. Jahrbiicher, Bd. 8, p. 445. 



143. The Sieve Tubes and their Functions in Connection with 

 Translocation. 



If we cut through the stem of a plant of Cucurbita, a consider- 

 able quantity of a mucilaginous fluid springs from the cut surface. 

 On consideration of its quantity, it is at once clear that the sap is 

 forcibly driven out under the influence of pressure, and in fact 

 causes for such pressure effects are to be found in the organism, 

 as we shall see below. We will first examine the sap. 



We cut through the stem of a Cucurbita, e.g. C. Pepo (I used 

 C. minensis). We now touch the cut surface with a small piece 

 of red litmus paper, and find to our surprise that it becomes blue. 

 At all events, therefore, a large part of the sap flowing from the 

 stem has a comparatively strong alkaline reaction, whereas most 

 plants when they have been wounded yield a sap acid in reaction, 



