156 TRANSLOCATION IN PLANTS 



would seem insufficient to account for a low turgor, espe- 

 cially when the receiving tissues are in direct contact 

 with water and the supplying tissues are not, as was the 

 case with the onion when the receiving roots were sub- 

 merged in water and the storage tissues were in the air. In 

 spite of the evidence to the contrary some may think 

 sugar transport from storage tissue may take place with 

 the transpiration stream. This could hardly be involved 

 to account for transport from the flaccid onion scales to the 

 turgid roots submerged in water. 



For the most part Miinch assumes that water is excreted 

 by the receiving tissue and this excretion is due to the 

 pressure resulting from a flow of solution into the cells. 

 Under such circumstances the receiving cells must be at 

 their maximum turgor and must have a concentration 

 less than that of the supplying cells where the turgor 

 pressure is initiated. These osmotic concentration meas- 

 urements seem definitely to refute the Miinch hypothesis. 



b. Even if the supplying cells of photosynthesizing tissues 

 which have not been extensively tested should have 

 osmotic concentrations greater than those of receiving 

 cells, this would not insure a pressure gradient leading 

 toward the receiving cells, because the turgor of the exposed 

 leaf may be greatly reduced, especially during periods of 

 high isolation. In fact the turgor of the leaf may be 

 reduced to such an extent on a bright day as to reverse 

 the pressure gradient. This should lead to movement 

 into the leaf instead of out of it. Without going to such 

 an extreme the reduced turgor in the leaf should reduce 

 the rate of transport during the day. The available 

 data, however, clearly indicate greater transport during 

 the day than at night. Mason and Maskell (1928a) found 

 greater transport into the cotton boll during the day, as 

 indicated by a gain in dry weight during the day about 

 four times that during the night. Crafts (1931) estimated 

 transport from the leaves of Phaseolus during the day to 

 be a little over three times that in darkness. Data on 

 relative turgors of supplying and receiving tissues under 

 such conditions are not yet available. 



