TRANSLOCATION OF ORGANIC FOOD-SUBSTANCES 575 



Every tissue has, however, a certain power of translocation, and no 

 doubt different parenchymatous tissues exhibit varying degrees of functional 

 activity and differentiation. Thus large quantities of reserve-materials are 

 rapidly transferred to the developing embryo through parenchymatous 

 endosperm, and in young seedlings the further transference probably takes 

 place mainly through the cortical and medullary parenchyma. This is all 

 the more probable because the distances to be traversed are small, and 

 the relatively large sectional areas of the parenchymatous tissues com- 

 pensate for the slower rate of transference. The extent to which plastic 

 substances are, or may be, conveyed through the cortex, medulla, &c., 

 has in no case been precisely determined, but in the stems of seedlings, and 

 in shoots, branches, &c., whenever reserve-materials are being consumed or 

 stored, chains of parenchyma cells in the intermediate regions of the .cortex, 

 and of the medulla as well, are usually found filled with starch, sugar, oil, 

 asparagin, &c. It is, however, impossible to say whether these tissues serve 

 as direct channels or simply as temporary storage reservoirs, in which any 

 excess is stored for a time and later returned to the phloem for further 

 transference. Exchanges of this kind are of general occurrence between 

 the conducting channels and the storage tissues. Thus in trees the assimi- 

 latory products from the leaves are conveyed to the main trunk by the 

 phloem and there distributed horizontally by the medullary rays, whence 

 they may again be transferred to the conducting phloem tissues. Similarly it 

 is in the parenchyma immediately surrounding the phloem that the trans- 

 located substances first appear, and when the amounts translocated are 

 small a perceptible accumulation may only occur at this point. In such 

 cases the transference takes place in the transverse direction through the 

 parenchyma, but there is no reason why longitudinal conduction should 

 not also be possible. Indeed translocation is in general more rapid when 

 the parenchyma cells are elongated in a direction parallel to the length of 

 the stem, and hence also to the phloem elements. 



It is probable therefore that in seedlings, and frequently also where the 

 distance to be traversed is small, the cortical and medullary parenchyma 

 may be concerned to a greater or less extent in the transportation of 

 all or of single nutritive or excretory substances. Schimper 1 indeed has 

 found that after the removal of the vascular bundles from the leaves of 

 Plantago the bundle-sheaths are still able to convey the assimilatory 

 products to the stem, though somewhat less rapidly. An accumulation of 

 starch, sugar, &c. observed in a tissue during translocation does not neces- 

 sarily show that the tissue in question forms an active conducting channel, 

 for accumulation involves retention, whereas in the actual channels the 

 transference may be so rapid that no perceptible quantity of the trans- 



1 Schimper, Bot. Zeitnng, 1885, p. 756. . Cf. also Czapek, 1. c.. p. 126. 



