K.— BOTANY 207 



with a vigorous basipetal growth and differentiation of the phloem, which, 

 Hke the earHer differentiation of spring wood, begins in the leafy shoot and 

 spreads from thence downwards over the axis. The successive enlarge- 

 ment and division of cells that lie below one another in the cambial 

 cylinder, which must take place during this new formation of 

 phloem, represents in itself a very considerable downward movement of 

 food. 



The following very approximate calculation may be presented in this 

 connection. In Fraxinus excelsior the structure of the phloem makes it 

 practicable to remove it over small areas in fairly smooth tangential 

 sheets. The fresh weight of a square centimetre of such a sheet, separated 

 from the tree in April and containing the phloem formed in the two 

 previous growing seasons, proved to be about 0-046 gram ; the dry 

 weight o-oi8 gram. This phloem was taken from a small tree perhaps 

 twenty to thirty years old, from the short main trunk which possessed 

 about 14,000 sq. cm. of surface. The dry weight added to this trunk by 

 the formation of phloem during one season's grovrth would then be of the 



order of -^ = 126 grams. Ramann and Bauer found that the 



2 



increment of dry weight in the stem in one growing season, in 100 two- 

 year-old ash trees, was about 2,437 grams, so that a single tree gained 

 about 24 grams. In the older tree the increment of weight would be 

 much greater, but when it is remembered that to the gain of weight due 

 to the formation of phloem in the trunk has to be added that due to the 

 new phloem on all the branches together with the formation of the thick- 

 walled summer wood throughout the stem, then it would appear that a 

 large proportion of the downward movement of organic solutes is effected 

 during the actual growth and differentiation of these tissues, in which case 

 the mechanism of movement would be closely linked with the basipetal 

 mode of growth of the cambium. 



So long as the cambium is still growing and differentiation proceeding, 

 the downward movement of organic material in the tree must be closely 

 connected with these growth processes, and no mechanism of transfer 

 which is independent of them can accurately represent the processes at 

 work. It may be that subsequently, in fully differentiated sieve tube, 

 companion cell, etc., translocation of food still takes place, but on the other 

 hand, the structural features of the adult sieve tube may rather be analogous 

 to those features in a dry river bed which supply evidence that it was once 

 a channel along which a rapid current flowed. 



This brief review of some of the many problems presented by the form, 

 structure and function of the growing tree has been presented, so far as 

 possible, upon very general lines in an attempt to show that the issues 

 thus raised, if primarily botanical, yet make a very wide appeal to our 

 interests. A more detailed discussion of most of these problems will be 

 found in a series of papers ^ in which citations of literature are given. 

 It is hoped that this general statement has shown that the study of the 



^ ' Studies in the Physiology of Cambial Activity,' New Phytologist, 29, 1930. 



