232 TRANSLOCATION IN PLANTS 



ported through sieve tubes and that its movement is correlated with a 

 diffusion gradient. 



35. Most of the earUer data that might be considered as offering evidence 

 on concentration gradients are of doubtful significance because they con- 

 sisted chiefly of mass analyses of leaves and stem tissues in which the xylem, 

 phloem, and cortical regions were not separated. Some also were expressed 

 as percentages of dry weight which cannot be safely interpreted in terms 

 of concentration gradients. Mason and Maskell, by analyses of bark 

 tissue subdivided into outer, middle, and inner regions so as to compare 

 parts with a high cortical parenchyma content with those having a high 

 phloem content, found the cortical parenchyma to be rich in hexoses and 

 low in sucrose, while those tissues that consisted chiefly of phloem were rich 

 in sucrose and poor in hexose. The vertical gradients for sucrose were 

 always in the direction of actual sugar transport, independently of whether 

 the direction was natural or reversed by various treatments. The gradient 

 of sucrose led to the receiving boll, while that for hexoses led from the boll 

 to the bark. This would seem to lead to a transport from the boll of hexoses 

 unless some mechanism is effective in preventing such loss. Such a mechan- 

 ism is assumed to exist. Phillis and Mason give strong evidence showing 

 that the phloem is capable of absorbing sugar from the mesophyll under 

 conditions that build up a steep reverse gradient of sucrose leading back 

 to the mesophyll. The attempts of Maskell, Mason, and Phillis to deter- 

 mine concentration gradients for nitrogen have been less successful. These 

 determinations are comphcated by the fact that nitrogen is present, in 

 many forms of which some may be moving while others are static, and by 

 the fact that the forms of nitrogen may quickly change from one to another. 

 Another still more serious weakness which they overlooked lies in the fact 

 that these investigators have not established with any degree of certainty 

 the direction in which nitrogen is actually being transported. They assume, 

 with almost no evidence to support the assumption, that all nitrogen first 

 moves entirely through the xylem to the leaves, that the phloem receives 

 none from the roots and none from the xylem, and that all which is moving 

 through the phloem has come entirely from the leaves. 



36. Van den Honert has demonstrated that a substance that lowers the 

 surface tension between two immiscible liquids will spread rapidly over the 

 interface; and he suggests that sugars may be transported in phloem tissues 

 by a similar mechanism. The direction of such movement is determined by 

 the concentration gradient, and the rate of movement seems to be of the 

 same order as that calculated for sugar transport through the phloem. He 

 suggests that movement of protoplasm may be the result of transport and 

 not the vehicle. This rapid-moving surface film may be considered as a 

 special type of protoplasmic streaming. 



37. Schumacher has reported a rapid transport of fluorescein through the 

 sieve-tube system. This dye does not appear in the vacuole but is seemingly 

 absorbed and transported exclusively in the protoplasm. When introduced 

 through leaves, the direction of movement is nearly always toward the stem 

 from where it may move either down or up or in both directions. When 

 moving up, it rarely passes into leaves, even into the young ones that are 

 but half grown. Rates up to about 5 to 6 mm. a minute were observed at 



