TRANSLOCATION OF CARBOHYDRATES 271 



ultimately a pad of callus is formed on either side of the plate. 

 Traces of the pores may sometimes be seen, such as are figured 

 by Hill, who suggests that in this manner the streaming of the 

 assimilates is checked and the sieve-tubes sealed until the 

 following spring. Then, by the dissolution of the callus, the 

 pores are again opened and the sieve-tubes resume their con- 

 ducting function. In other plants the sieve-tubes are only active 

 during a single year, after which they lose their living proto- 

 plasm and are replaced functionally by new elements. 



It will be seen from the above account of the sieve-tubes that 

 their anatomical structure and their relations to the surrounding 

 tissues fit them admirably for the rapid translocation of the 

 assimilates as a whole ; in the absence of any critical proof 

 to the contrary it seems illogical to deny that the sugars follow 

 the same paths as the albuminous substances, which it is admitted 

 travel in the sieve-tubes. I hope before long to publish, an 

 account of experiments which have shown that sugars actually 

 do travel in large quantities in the sieve-tubes. 



The Leaf 



The structure of the leaf, in which assimilation is most active, 

 may now receive closer attention. Essentially it is composed of 

 layers of chlorophyll-containing cells so arranged as to expose 

 a large surface of tissue kept extended by means of ribs, veins 

 or nerves, as they are variously termed. The larger veins 

 generally unite at the base of the leaf or pass into a midrib 

 continuous with the leaf stalk, the tissues of which in turn are 

 continuous with those of the stem, except in certain cases to be 

 mentioned presently. Thus it is clear that the veins and stalks 

 of leaves are particularly suitable for observations upon trans- 

 location, since all the assimilates must pass through them on the 

 way to their destinations. 



The lamina of the leal shows structural indications of the 

 path of the assimilates and was studied in detail by Haberlandt 

 in 1882. Some ten types of leaf structure were distinguished 

 by him and classified according to the degree to which they 

 appeared to express the principle of removing the assimilates 

 by the shortest possible path. Generalising from these types it 

 may be said that the lamina is bounded by an upper and a lower 

 epidermis in one or both of which stomata are present. Except 



