THE METHOD OF MOVEMENT 233 



temperatures around 30°C. These rates were greatly reduced at around 

 11°C. and they became barely perceptible below 4°C. The rate seems 

 entirely uninfluenced by anesthetics, and is independent of the movement of 

 carbohydrates or nitrogen. He suggests that, although the transport of 

 fluorescein takes place in the protoplasm, its movement is not dependent on 

 moving protoplasm. The evidence for this latter conclusion is not strong. 

 Although the movement of fluorescein through phloem tissues may be 

 strictly comparable to that of sugars and other substances, it is possible 

 that sugars may move by a different mechanism. The quantity of sugar 

 transported seems immensely greater than that of fluorescein or growth 

 substance. 



38. Various hormones and viruses may or may not be transported in the 

 same tissues and by the same mechanism concerned in food transport. 

 There is evidence that growth substances are electrically charged and show 

 strict polar movement only. Went has suggested that these substances 

 are carried cataphoretically and that the movement takes place in companion 

 cells. The evidence for the latter point is very weak. Van der Wey has 

 given considerable evidence that transport of the growth substance of Avena 

 coleoptiles is strictly polar; the velocity of transport is almost independent 

 of temperature, but the amount transported is greatly influenced by tem- 

 perature. The substance is supposed not to be carried by moving proto- 

 plasm and he suggests that it is carried in the protoplasm lying next to the 

 walls as well as in that permeating them. There are similarities as well as 

 striking differences between the reported transport of growth substance, 

 of virus, of fluorescein, and of sugar. All are carried in living cells and seem 

 to be under partial control of them. The velocity of movement of growth 

 substance is about the same as that of fluorescein moving through paren- 

 chyma cells, and much less than that of fluorescein moving in sieve tubes by 

 20 to 100 times. Little is known about the normal velocity of sugar move- 

 ment, although it is probably more nearly the same as that of fluorescein. 

 The velocity of movement of growth substance seems almost uninfluenced 

 by temperature while that of fluorescein is markedly influenced. Evidence 

 is lacking concerning the influence on velocity of sugar transport. Move- 

 ment of growth substance, at least in coleoptiles, is strictly polar, that of 

 fluorescein and virus is mostly polar in leaves but not so in stems, while that 

 for sugar is probably mostly nonpolar. The amounts of growth substance 

 or virus normally carried are extremely minute, those of introduced fluorescein 

 may be somewhat greater, while sugars are normally transported in great 

 quantity. The evidence is inadequate to make clear-cut comparisons, but it 

 seems to indicate that, in transporting the different types of substances here 

 mentioned, different tissues (parenchyma, phloem sheaths, or phloem 

 proper), different parts of the same tissue (sieve tubes, companion cells), 

 or even different regions of a cell (vacuole or specific layers of protoplasm 

 in sieve tubes) may be involved. It is also possible that the mechanism 

 of transport may vary with the kind of material transported and with the 

 tissue of transport. 



39. Although denied by some the evidence is rather conclusive that 

 appreciable transport does not take place through dead phloem. There 

 is considerable evidence that anesthetics interfere with transport of sugars, 



