K.— BOTANY. 197 



have passed in 100 days. Therefore the average velocity of flow through 

 this conduit, having a cross-section of 0. 0042cm. ^, must have been 



0.0042x100x24 ' '•'• ""^"^^ ^°""'- P^"^ ^°"''- 



By the second method we an-ive at a different figure. Various 

 investigators, from Sachs onwards, have measured the rate of photo- 

 synthesis per square metre of leaf per hour. Under the most favour- 

 able conditions the amount may approach 2g. , and it has been esti- 

 mated as low as 0.5g. Taking Brown and Moi'ris' determination for 

 Tropceolum viajus, viz., Ig. per square metre per hour, and assuming 

 one- third of the caAohydrate formed is used in respiration in the leaf, 

 we find tliat a leaf of 46cm. ^ may form dming ten hours' sunshine 

 0.46g. ; during the twenty-four hours one-third of this will be respired, 

 leaving O.Slg. to be transported from the leaf. The volume of the 

 solution (again assuming a concentration of 10 per cent.) will be 

 3. 10cm. ^. The cross-section of the bast of the bundles in the petiole 

 vi^as 0.0009cm. ^, therefore the velocity of flow, if the bast was used as 



3 10 



the channel of transport, must have been pr-—^ — ki or 140cm. per 

 ^ 0-0009x24 ^ 



hour. 



Similar figures to these were derived using measurements obtained 

 from a number of potato-tubers and from various leaves. The veloci- 

 ties indicated, even assuming a concentration of 10 per cent., lay in 

 all cases between '20cm. and 140cm. per hour. These figures are in 

 agreement with those arrived at by Luise Birch-Hirschfeld, as to the 

 weight of organic material transported from leaves. 



A flow of this rate through the bast seems quite impossible. The 

 narrow transverse section of its elements, the frequent occurrence of 

 transverse walls, and the lining of protoplasm and large protein contents 

 practically preclude the mass movement of liquid through this tissue. 

 If we imagine the flow restricted to the sieve-tubes the velocity must 

 be correspondingly increased, and the excessively fine sieve-pores, more 

 or less completely occupied by colloidal proteins, must be reckoned 

 with. Simple diffusion, as Czapek recognised, cannot account for the 

 transport, and there is no reason to suppose that adsorption on the sur- 

 faces of the colloid contents of the sieve-tubes can increase the velocity 

 of diffusion, as Manghan suggests. 



As soon as one realises the volume of the solution which has to 

 be transported, and the velocity of the flow that this necessitates, one 

 naturally turns to consider if the open capillaiy tubes of the wood may 

 not be utilised as channels of transport. Deleano's results, indicating 

 that the depletion of leaves continues even after the living elements of 

 their petioles have been killed, support this conjecture. 



The emphasis wHich has been laid on the function of the wood 

 as providing a channel for the upward movement of water usually 

 obscures its function as a downward and backward channel also. Early 

 experimenters, however, fully recognised that, under certain conditions, 

 the current in the wood may be reversed. Thus Hales quotes several 

 experiments of his own proving this point, and states that some of his 



