Chapter IX — 175 — Uptake and Movement 



water. From the pictures that have been presented in the preceding chapters of the 

 molecular structure of water, carbohydrates, amino acids, and proteins, it seems that the 

 mutual attraction of these substances, as indicated by the various points available for 

 coordination through hydrogen bonds, would resist the movement of food substances 

 either along the surface of the protoplasm or through static water. Those molecules 

 that were held in the protoplasm would be removed only with difficulty. Those in the 

 solution occupying the lumina of sieve tubes would diffuse independently over short 

 distances to satisfy local gradients ; their rapid movement over great distances inde- 

 pendent of the solvent is not possible by any known physical mechanism. 



Although the rapid migration of hydrogen and hydroxyl ions has been pictured as 

 involving a proton jump along chains of coordinated water molecules, a similar move- 

 ment of large molecules seems improbable. 



The mass-flow mechanism is supported by evidence for a permeable condition of 

 the functioning sieve tube, the presence of a concentration gradient of osmotically active 

 materials in the phloem and the phenomenon of phloem exudation that seems, at least 

 in some instances, to account for normal rates of flow. It receives substantiation from 

 studies on the movement of virus, auxin, and radioactive indicators that depict a simul- 

 taneous transport of foods and indicators, depending largely upon the establishment of 

 source and sink by regulating assimilation or hydrolysis at the source and utilization 

 or storage at the sink. Weaknesses of the mass-flow theories include a lack of 

 knowledge relative to the resistance offered by cross walls in the phloem, need for more 

 convincing evidence for the passive function of sieve tubes, a fundamental discrepancy 

 in the composition of phloem exudate from cucurbits as contrasted with composition of 

 their fruits, and a general lack of analytical data on the sieve-tube contents of a variety 

 of plants. Cucurbits which have been employed so much have proved atypic (Crafts 

 and LoRENZ, 19446) and should be used with caution. Future work, to be of value, 

 should make use of viruses, auxin, and radioactive isotopes that can be injected with 

 little or no injury and detected by very delicate and accurate methods. 



Reviews on translocation cover the literature quite thoroughly up to about 1940; 

 Curtis (1935) and Mason and Phillis (1937) emphasize the viewpoint of the 

 protoplasmic theories; Crafts (1938, 19396) supports the mass-flow hypothesis. More 

 recent publications on the relation of the structure of the sieve tubes to their function 

 (Crafts, 1939a, c) extends the view that these elements become permeable at maturity 

 allowing ready flow of nutrients in solution through the phloem. Observations on 

 phloem exudation are extended. Clements (1940) and Cooil (1941) question the evi- 

 dence for the lowered activity of mature sieve tubes and the attendant high permeability. 

 Crafts and Lorenz (1944a, 6) found translocation into the fruits of cucurbits to 

 be very rapid but concluded that phloem exudation in these plants could not be accepted 

 as a manifestation of normal phloem transport. The carbon : nitrogen ratio of the 

 exudate was not compatible with that of the fruits. 



Analyses of the stems and leaves of raspberry by Engard (1939a, 6) support the 

 view that transport of organic materials follows a diffusion pattern, gradients being 

 present where translocation occurs. Major carbohydrate movement is in the form 

 of sucrose and occurs in the phloem. Nitrogen absorbed by the roots moves upward 

 freely in the xylem and is available for reduction and elaboration to amino acids and 

 protein in all living cells of the plant. 



LooMis (1945) from many studies on carbohydrate metabolism in maize con- 

 cludes that sucrose is the principal compound involved in translocation. He considers 

 that neither the protoplasmic nor mass-flow mechanism can explain carbohydrate trans- 

 port in maize and proposes a mechanism involving polarized movement, a secretory 

 type of movement that commonly takes places against gradients in concentration. 

 Went and Carter (1945) show that injury of the petioles of tomato plants hinders 

 sugar transport through the adjacent stem. 



Indicators of various types have been employed in translocation studies. Rouschal 

 (1941) used fluorescein and found that it tends to accumulate in young active cells and 

 in the protoplasm of mature sieve tubes but not in their lumina. The dye streams 

 along with assimilates in the phloem and this flow can be reversed, hence he pictures a 

 mass-flow mechanism. He found that whereas the main flow of the assimilate stream 

 from a leaf occurred in the large sieve tubes of the petiole, independent and opposed 

 streaming may take place in extrafascicular sieve tubes. Thus movement both in and 

 out of a given leaf might occur by mass flow. Such movement might also take place 

 in opposite directions in the stem in different vascular bundles. 



Radiophosphorus has been used as an indicator. Biddulph (1940) found this ele- 



