194 SECTIONAL ADDRESSES. 



the wood is unable to transmit sufficient supplies of organic substance. 

 As Strasburger has already pointed out, this interpretation rests upon 

 the fallacy of supposing that the removal of the bark as far as the 

 cambium leaves the wood uninjured. As a matter of fact, microscopic 

 examination of the wood, from which the outer tissues have been 

 stripped, shows that its trachefe soon become blocked with air-bubbles 

 and with substances probably exuded into them and their walls during 

 morbid changes in the cells of the cambium, in the cells of the medullary 

 rays, and in those of the wood-parenchyma. The blocking is accom- 

 panied with discoloration, and is most apparent in the outer layers 

 of the wood. It is only reasonable to suppose that the efficiency of 

 the tracheae as channels of transmission is seriously impaired even 

 before there is visible evidence of plugging. 



It is evident that this clogging may act differentially on the water 

 and the substances it carries in it. In the first place, the whole cross- 

 section of the wood is available for the transport of water, while 

 probably the outer layers are mainly utilised by the organic substances. 

 Further, colloidal deposits in the walls, and especially in the pit- 

 membranes, would obstruct the passage of organic substances much 

 more than they would the water which carries them. These considera- 

 tions readily explain how it is that, while the water-supply to the buds 

 of ringed branches is adequate, the supply of organic substance may 

 be deficient. 



Apart, then, from the very slow movement of organic substances 

 from cell to cell, there is very cogent evidence that their upward motion 

 is effected in the trachese of the wood. There is no reason to believe 

 that during this transport the walls or pit-membranes of these tracheae 

 oppose the passage of the dissolved carbohydrates or of the simpler 

 proteins any more than the water which conveys them. Hence the 

 velocity of transport of these organic substances is that of the transpira- 

 tion current, and the amount conveyed in a 'given time depends on the 

 velocity and concentration of the stream. 



The transport of organic substances in an upward direction in plants 

 is secondary, for, as is well known, carbohydrates certainly, and pro- 

 teins most probably, are manufactured only in the upper green parts of 

 plants — principally in the leaves, and must be transported in the first 

 instance back from these to the stems to be distributed to the growing 

 regions and to the storage organs. 



It is almost universally held that the channel for this backward or 

 downward motion of organic substances from the assimilating organs 

 is the bast of the conducting tracts. The orthodox position is summed 

 up by Strasburger as follows : In woody plants the carbohydrates manu- 

 factured by the leaves pass downwards in the bast. Movements of 

 carbohydrates in the opposite direction in this tissue only take place if 

 they are occasioned by local consumption. From the bast the carbo- 

 hydrates spread into the medullary rays and wood-parenchyma, and in 

 young branches fill these tissues and more or less of the pith. A down- 

 ward movement in the wood-parenchyma, such as was formerly held, 

 does not take place, and in those conifers in which there is no con- 

 tinuous wood-parenchyma is anatomically impossible. In spring and 



