THE METHOD OF MOVEMENT 187 



that this virus moves rather slowly and may move exclu- 

 sively through parenchyma cells and not through the 

 phloem. 



Van der Wey (1932) has given almost incontrovertible 

 evidence that the growth substance of Avena coleoptiles 

 is carried in one direction only and he gives strong evidence 

 that its transport is independent of protoplasmic streaming. 

 This growth substance is probably carried through phloem 

 parenchyma cells of the coleoptile, but whether it is also 

 carried through sieve tubes is not known. 



Went (1932) has proposed that specific substances, such 

 as these growth substances, are carried cataphoretically 

 under the influence of a constant electrical gradient in the 

 plant. He suggests that these are carried in the companion 

 cells, while the foods are transported through the sieve 

 tubes by a pressure mechanism, such as that proposed by 

 Miinch. Although one might expect growth substances 

 to be carried through the phloem by the same mechanism 

 as food transport, this point is still far from settled. 



Schumacher (1933) has observed that fluorescein moves 

 almost exclusively in a basipetal direction through leaf 

 petioles of Pelargonium, although when reaching the stem 

 it more commonly spreads in both directions. Critical 

 experiments clarifying this point, as to whether transport 

 of solutes through the phloem is strictlj^ unidirectional or 

 not, would help greatly in our solution of the problem as 

 to the possible mechanism of transport. (For further 

 discussion of transport of special substances see Sec. 38.) 



e. Another weakness in the streaming hypothesis lies 

 in the possibility that movement might not be determined 

 by a diffusion gradient. Whether this weakness is to be 

 considered serious or not, will depend upon the outcome 

 of investigations which should reveal the fact whether 

 transport does or does not follow a diffusion gradient. 

 Thoroughly adequate data on this point are at present 

 lacking, although recently INIason and Maskell have 

 presented rather conclusive evidence that sucrose is the 

 sugar of transport and its movement through the sieve 



