ABSORPTION AND MOVEMENT OF WATER 159 



The presence of starch-grains in the sieve-tubes has given 

 occasion to the hypothesis that either the sieve-tubes them- 

 selves, or their companion-cells, are the chief elaborators of 

 carbohydrates into nitrogenous matters.* Such is, how- 

 ever, hardly likely to be the fact (p. 69 ). Although very 

 likely the sieve-tubes do form nitrogenous food from nitrates 

 and carbohydrates, this function seems not yet to have 

 been appropriated by any special tissue. Apart from the 

 classical experiments of ringing or girdling plants, with the 

 result that the downward transfer of food is nearly or quite 

 stopped, it must be admitted that exact experiments and 

 definite knowledge on the special functions of the sieve-tubes 

 remain for the future, t The recent discovery by Raciborski J 

 of a substance resembling the haemoglobin of higher ani- 

 mals in that it readily gives up oxygen to the sieve-cells 

 and lactiferous tubes, in which it regularly occurs, suggests 

 that besides conducting and perhaps contributing to the 

 elaboration of foods, these tissues may also obtain needed 

 oxygen from the substances which they conduct. But as 

 Raciborski says in his second paper, this discovery made in 

 the tropics and away from laboratories must be tested and 

 made the starting-point for research by plant-physiologists 

 with laboratory facilities. 



The milk-tubes or lactiferous ( laticiferous ) tubes or ves- 

 sels, occurring in a very considerable number of plants and 

 forming continuous systems often as extensive as the sieve- 

 tubes, are filled with a mixture of the most diverse com- 

 pounds dissolved or suspended in water. Some at least of 

 these substances are the often very useful by-products 

 formed in nutrition, respiration, or in other vital processes. 

 On the other hand, some being subjected to more or less 

 profound chemical changes, serve as sources of energy in 

 respiration, as materials for the construction of cell- wall, 



* Sachs. J. von. Lectures on the Physiology of Plants. Engl. transl.. 

 p. 325. 



f Trelease (Sixth Annual Report. Missouri Bot. Garden. 1894) finds no 

 sieve-tubes at all in Leitnerin floriflana. 



i Raciborski. M. Ein Inhaltskorper des Leptoms. Ber. d. Deutsch. Bot. 

 Gesellsch.. Bd. XVI.. 1898. Weitere Mittheilungen iiber das Leptomin. ibid. 



