TRANSLOCATION OF CARBOHYDRATES 259 



external air into tiie interior of the leaf) per unit area of the 

 leaf. Further, the work of Brown and Escombe (1901) upon 

 the passage of gases through " multi-perforate septa " removed 

 the objection that the stomata were inadequate to take in the 

 large volume of air required to furnish the carbon dioxide in 

 sufficient quantity. They showed that a septum with holes at 

 not more than a certain number of diameters apart allowed 

 diffusion to proceed almost as freely as if no septum existed. 



But before these more recent results were obtained it became 

 evident that during exposure to sunlight, the leaves form 

 organic compounds, and a movement or translocation of these 

 had to be assumed in order to explain growth in distant parts. 



The problems remaining after the importance of the leaves 

 was realised were to ascertain the chemical nature of the 

 assimilates, the conditions and places of their formation, the 

 causes of their movements and transformations and the paths 

 followed by them in their passage from the leaves. Here the 

 only problem to be considered in detail is the last one, which 

 became more interesting when microscopic studies revealed to 

 Theodor Hartig, in 1837, the existence of sieve-tubes — long 

 chains of cells whose contents communicate by means of pores 

 in the transverse walls. (Figs. 3-8.) 



The actual substances translocated consist of albuminous 

 and of non-nitrogenous compounds. The latter are for the most 

 part carbohydrates and will here receive particular attention. 



It has been supposed by some that to a great extent these 

 two classes of compounds travel from the leaves by separate 

 paths. The less diffusible albuminous substances were thought 

 to pass along the sieve-tubes, while the more easily diffusible 

 compounds were thought to be conducted adequately by the 

 less specialised, thin-walled cells forming the general tissue in 

 which the vascular strands of the conducting organs lie. When 

 such cells are elongated the tissue has been called "conducting 

 parenchyma." (Fig. 2.) 



Nageli first enunciated this view as to the function of sieve- 

 tubes (1861) and the theory of separate paths has on the whole 

 been the favourite one. 



The anatomical studies of Haberlandt (1882) led him to put 

 forward this view more definitely and the same theory was 

 supported by Schimper (1885). The latter endeavoured to show, 

 by microchemical and other means, that the sugars travel from 



