Chapter IX — 177 — Uptake and Movement 



movement in the direction which food materials might be expected to move. Studies 

 on pantothenic acid and riboflavin (Bonner and Borland, 1943) indicated that distri- 

 bution of these compounds follows a pattern similar to that of thiamin. Pantothenic 

 acid accumulated at girdles in the same way as thiamin; riboflavin failed to accumu- 

 late. 



In a third series, Bonner (1944) tested large lots of tomato plants by girdling 

 some and leaving others ungirdled and measuring concentrations initially and finally 

 present in both girdled and ungirdled plants. He found that thiamin, pyridoxine, panto- 

 thenic acid, riboflavin, sucrose, total nitrogen, and non-protein nitrogen all accumu- 

 lated above the girdle. The extents and rates of accumulation varied and Bonner 

 questions the value of such evidence as it relates to the mechanism of mass flow. In 

 view of the possible differences in rate of synthesis, rate of transport, degree of accumu- 

 lation, utilization in growth, destruction, and possibly leakage into the xylem, it seems 

 unlikely that uniform rates and ratios of accumulation could be expected. The evi- 

 dence, such as it is, indicates a simultaneous movement of these various materials in 

 the tomato, and such movement can best be explained by mass flow. 



Recent work on translocation of plant hormones has concerned the effects of 

 dichlorophenoxyacetic acid on weeds. In 1944, Beal found that the roots of sweet pea 

 produced nodule-like swellings and that the stems bent and grew abnormally following 

 application of 4-chlorophenoxyacetic acid to the stem or leaflets. Evidently this material 

 moved within the plant. 



Ferri (1945), studying translocation of synthetic growth substances, showed that 

 they could be absorbed by roots from the soil and moved upward through the xylem. 

 Such movement was independent of living cells. Similar experiments were performed 

 by Skoog in 1938. 



Recent work on absorption and translocation of 2,4-dichlorophenoxyacetic acid 

 by Mitchell and Brown (1946) shows that movement of the stimulus resulting from 

 treatment is correlated with movement of organic foods in bean plants. When the 

 2,4-D was applied to roots the resulting stimulus was translocated through non-living 

 cells of the stem, indicating that it traveled in the transpiration stream in the xylem. 

 Weaver and De Rose (1946) obtained similar results with bean plants. In alfalfa, 

 basipetal movement was most rapid in young active tissues. Figures 1 and 2 of their 

 paper give excellent evidence for upward movement in the xylem past a killed region 

 and subsequent accumulation in the phloem. This is reminiscent of the results of 

 Maskell and Mason for nitrogen and of Skoog for auxin. 



From this review it is evident that the tendency is toward use of non-toxic and 

 naturally occurring indicators in translocation studies. The more recent work sup- 

 ports the view that such indicators, when in the phloem, accompany any food materials 

 in their movement through the plant. The simplest and most compatible mechanism 

 to account for such flow would seem to be mass flow along hydrostatic gradients de- 

 veloped osmotically. Such a mechanism involves a recirculation of water and entails 

 a close correlation between the functioning of xylem and phloem. A thorough con- 

 sideration of this mechanism in all of its ramifications brings out the fact that practically 

 every plant function is in some way tied in with processes of solute and water move- 

 ment. Water is absolutely essential to the welfare of higher plants and as such plants 

 have left their moist habitats and invaded the valleys, the mountains, and the deserts, 

 they have developed intricate absorption and translocation mechanisms and marvelous 

 protective coatings. By means of these they have maintained their aqueous sur- 

 roundings to a remarkable degree ; that is, they have taken their environment with 

 them. And if we, as agriculturists, wish to further their welfare and so protect our- 

 selves, we must, to the limit of our abilities, improve the environments of our crop 

 plants. We must do to a greater degree what the plant has so largely done for itself, 

 that is, we must simulate the perfect environment for plant growth and production by 

 supplying, along with minerals and a favorable soil, an adequate and unfailing supply 

 of water. 



Summary: — Simple plants having no vascular tissue are limited to moist habitats; 

 higher plants have developed tissues for rapid uptake and transport of water ; protective 

 layers restrict water loss; these plants survive dry habitats whilst their living cells 

 have an aqueous medium. The soil acts as a reservoir from which the plant can absorb 

 so much water. 



Water may be absorbed actively from soils near their field capacity. In a saturated 

 atmosphere such absorption causes guttation. Under conditions of high transpiration 

 water loss exceeds uptake and a passive absorption by the roots takes place. 



