TRANSPORT OF ORGANIC SUBSTANCES 



IN PLANTS. 



ADDRESS TO SECTION K (bOTANY) BY 



Peofessor H. H. DIXON, Sc.D., F.R.S., 



PRESIDENT OF THE SECTION. 



Plant physiologfsts have not paid the attention to the transport of 

 organic substances which the importance of the subject appears to 

 deserve. The ascent of water in high trees in defiance of gravity strikes 

 the casual observer and forces him to speculation as to how it is 

 contrived; but the problem of the transmission of organic substances 

 throughout plants only forces itself on those who are more or less 

 conversant with some of the leading facts of vegetable physiology. 



Among physiologists the usually accepted view is that organic sub- 

 stances are distributed throughout the plant by means of the bast. 

 The wood also acts as a channel of distribution for these substances 

 to opening buds and developing leaves, especially in spring when root- 

 pressure is active. The sap of bleeding contains appreciable quantities 

 of these substances, and their distribution to the developing buds in 

 spring by means of the wood was recognised by Hartig and Sachs. 

 Fischer showed that even in summer many woody plants contain 

 reducing sugars in their wood. The sugar content of the wood is at 

 a maximum in spring, diminishes in summer, and is at a minimum in 

 winter; at the end of February it rises rapidly. There is a second 

 crest on the curve at leaf-fall Dr. Atkins and myself extended 

 Fischer's observations, and showed that appreciable quantities of 

 soluble carbohydrates, sucrose, hexoses, and even maltose, are found 

 in the tracheae of the stems and roots of many trees at all seasons of 

 the year. Being in the tracheae they must be carried from the lower 

 parts to the upper growing regions, including the cambium of the stem. 

 Samples drawn from different levels during the spring were found to 

 have a greater concentration at higher levels. The inflow of water 

 below, and consequent dilution, is probably largely responsible for this 

 difference. Towards the end of the season there is no marked difference. 



This upward transport of carbohydrates in the tracheae seems to be 

 accompanied with smaller amounts of proteins. Thus Schroeder 

 showed that the quantity of proteins in the bleeding sap rises and falls 

 with the quantity of sugar. 



This view that the rising current in the tracheae carries organic 

 substances in it and distributes them to the growing regions has lately 

 been impugned. It was pointed out that in many cases ringing close 

 below the terminal bud prevents the development of that bud because 



