PHYSIOLOGICAL 315 



leaves. But just as the tracheae, or wood- vessels, are aided by the 

 tracheid cells, the wood-fibres, so the sieve-tubes of the phloem have 

 "companion cells" and "bast-parenchyma", often compared to side- 

 lines of transport. 



Many reasons have been given for regarding these complex phloem 

 strands as the conducting tissue for the synthetised foodstuffs in 

 solution, the "elaborated sap", as it used to be called. The observa- 

 tions show that the pith and the outer bark may be left out of 

 account as quite imessential for transport; the outer and younge- 

 wood-layers seem preoccupied with the transpiration current; 

 while experiments show that a cutting or blocking of the phloem 

 strands stops, or at least retards, the translocation of foodstuffs. 

 Moreover, a biochemical analysis of the coagulable and colloidal 

 contents of the sieve-tubes shows that they are peculiarly rich in 

 soluble foodstuffs, or, to be cautious, in complex carbon compounds. 

 This is also true of the contents of the latex vessels, whose milky 

 contents yield us opium, india-rubber, and gutta-percha, and thus 

 seem more suggestive of storage (if not sometimes also of riddance) 

 than of transport. An indirect argument in confirmation of the view 

 that the phloem strands form the main food- conducting system is 

 the fact that their development is exaggerated in cases where the 

 requirements for speedy transport are greatest, as in the stems of 

 vines extending for many yards, and in other prolonged shoots of 

 copious inflorescence. 



But while these and other arguments support this view, very 

 widely held, that the "elaborated sap" passes down the stem or 

 branch by the soft bast, and by its sieve-tubes especially, the recent 

 experiments of Prof. Henry Dixon and others point in another 

 direction. There seems to be more than the familiar up-current in 

 the strands of the young wood; for now these able investigators 

 give experimental evidence of a down-current in the wood, trans- 

 porting organic substances from the leaves. It is not denied that 

 materials may pass inwards from the phloem strands, and that 

 some of these pass into the ascending transpiration stream of the 

 wood; yet this seems to have its down-streams also; and there is 

 growing evidence that the materials thus borne down include 

 hormones — perhaps growth-controlling and metabolism-regulating 

 — besides ferments, which keep the wood-vessels in good working 

 order, as by rendering any starch-blockage soluble. 



STORAGE OF RESERVES.— It is characteristic of organisms 

 that they "accumulate energy acceleratively", as Joly many years 

 ago put it in his famous paper on "The Abundance of Life". A 

 growing leaf utilises part of the previously elaborated food before 

 it can go to work itself, and even has to make the first charge on its 

 own product of photosynthesis, but its increase of surface makes 



