TRANSLOCATION OF CARBOHYDRATES 269 



whose structure favours rapid longitudinal conduction are the 

 sieve-tubes ; in material collected in Ceylon by Mr. A. M. Smith 

 at the time when the fruit was nearly formed, these contained 

 a great deal of proteid material. 



Turning now to leaves, in which the assimilates are formed, it 

 has been shown by Lecomte that the phloem is more developed 

 in comparison with the wood than it is in the stem and still 

 more than in the root. He found too that relatively to the area 

 of cross section of the phloem strands the number of sieve-tubes 

 is greater in the leaf than in any other organ. Another way of 

 putting this result of Lecomte's investigation of leaf phloem is 

 to say that in tracing a phloem strand downwards from the leaf, 

 the elements other than sieve-tubes (fig. 8) increase in amount. 

 Now one of the most noticeable features of phloem is the absence 

 of intercellular spaces — all the cell walls are closely in contact, 

 even at the corners. Consequently the largest possible areas 

 of cell wall are secured through which lateral exchange of 

 material could go on. The relatively greater amount of paren- 

 chyma in the phloem of the stem and root than in the leaves can 

 be correlated with the physiological functions of this tissue. 

 The contents of the sieve-tubes have to be distributed through- 

 out the various portions of the plant and presumably the phloem 

 parenchyma plays a part in the lateral diffusion into the surround- 

 ing tissues. By way of these cells the sieve-tube contents pass 

 into the parenchymatous cells of the cortex and of the medullary 

 rays and into the wood parenchyma and pith. They may also 

 be stored in the phloem parenchyma itself. 



The distribution of the companion-cells suggests that they 

 may be the active cells which initiate the transference, whilst the 

 phloem parenchyma simply provides a pathway between the 

 sieve-tubes and the other tissues. Hence it is to be expected 

 that in storage organs, like stems and roots, the phloem paren- 

 chyma will be more abundantly developed than in organs like 

 leaves, which are mainly concerned with producing and despatch- 

 ing the assimilates as quickly as possible. Accumulation of 

 assimilates has been found to cause the stomata to close. Rapid 

 translocation is thus a biological advantage. 



Of the total assimilates which are removed from the lamina 

 by far the greater part consists of carbohydrates, which are 

 required to form the cellular framework of the plant, for storage 

 as reserves and for respiration. This fact makes it seem all the 



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