TRANSLOCATION OF CARBOHYDRATES 283 



of nutrition. The ramifications of the tubes beneath the epidermis 

 certainly provide a well-distributed supply of the closing fluid 

 and this is readily forced out when a hole is made owing to the 

 turgidity of the tubes containing it. As the tubes have a proto- 

 plasmic lining it may be that the sugars and small amounts of 

 mineral salts present simply serve to maintain turgidity osmoti- 

 cally and neither form a reserve nor represent assimilates in 

 course of translocation. It has indeed been suggested by Parkin 

 that the tubes are water-storage organs. However, it does not 

 yet seem possible to estimate their real value to those plants 

 possessing them or to say how much they assist in the conduc- 

 tion of food materials. 



Protoplasmic Connections 



It will be gathered from the above account of the anatomical 

 features of the various tissues dealt with, that in the cases of 

 the laticiferous tubes and the "conducting parenchyma" it is 

 difficult at first sight to pick out any one function as the primary 

 one. The construction of the bundle sheath and the tissue con- 

 tinuous with it in the main veins is such that the functions of 

 conduction, temporary storage and mechanical support are all 

 more or less probable. 



The sieve-tubes, on the other hand, cannot easily be conceived 

 to be well adapted for any other purpose than that of providing 

 a path for the rapid translocation of assimilates. Their structure 

 it is clear would enable such a movement to take place under 

 more favourable conditions than in the cells of the bundle sheaths, 

 for example, owing to the enlargement of the protoplasmic con- 

 necting threads. However, protoplasmic threads exist between 

 most cells in the plant and it will be well to devote some space 

 to a consideration of the significance of their occurrence. 



During the division of meristematic cells the nuclear sub- 

 stance of the two daughter cells appears to be connected for a 

 time by a number of fine protoplasmic threads arranged in a 

 more or less barrel-shaped bundle. These threads have been 

 thought by Gardiner to persist during the subsequent formation 

 of the cell wall between the two nuclei and so to unite the 

 protoplasts of the two cells. Though not fully established, this 

 theory of the origin of connecting threads is very plausible. 

 Whether the threads consist entirely of protoplasm, or whether 

 an interruption of another nature occurs at the " median node," 



