136 TRANSLOCATION IN PLANTS 



Any hypothesis explaining transport through the phloem 

 must take into consideration not only the structures of the 

 system through which transport takes place but also it 

 should be adequate to account for movement of carbo- 

 hydrates at rates which actually obtain. A number of 

 investigators, especially Birch-Hirschfeld (1920), Dixon 

 (1922), Mason and Lewin (1926), Tincker (1928), and 

 Crafts (1932, 1933) have reported calculations of rates at 

 which materials must be transported. 



Although until recently (Miinch 1926, 1927, 1930) no 

 adequate mechanism has been proposed to account for 

 unidirectional mass flow of solutions through the phloem, 

 it has been tacitly assumed by many botanists that such 

 movement of phloem contents does take place. Others 

 have definitely stated, or at least implied, that diffusion 

 accounts for the transfer through the phloem, but move- 

 ment by diffusion alone would be altogether too slow to 

 account for transport at the usual rates. 



27. Older Hypotheses and Suggestions. — Hartig (1858, 

 1860, 1861) had observed exudation from cut phloem and 

 assumed that there was a mass flow of sieve-tube contents. 

 NageU (1861) agreed with this and suggested that pressures 

 of neighboring cells caused the flow. Others suggested 

 that bending movements brought about by winds would 

 favor such flow. Sachs (1863) thought proteins might 

 move by such a pressure flow but that sugars moved 

 principally through parenchyma cells by diffusion. This 

 idea of diffusional flow through parenchyma was supported 

 by Schimper (1885). Although deVries as early as 1885 

 pointed out the complete inadequacy of diffusion alone to 

 account for movement, others have stated or implied that 

 it was adequate. Rywosch (1908, 1909, 1911) attempted 

 to support the diffusion hypothesis, but the evidence 

 presented is based largely on gradients in disappearance 

 or reappearance of starch, and to support it, phenomena 

 are cited which bear no relation to diffusion gradients. 

 Furthermore, although in the 1911 paper evidence is 

 given indicating that diffusion of glucose from a cell may 



