208 TRANSLOCATION IN PLANTS 



fluorescence. The fact that the nucleus shows lighting 

 before the cytoplasm, points to a gradual accumulation 

 which can be as easily explained on a basis of streaming as 

 by a rapid and almost instantaneous spread through the 

 cell. The fact that the rate of movement of the fluorescein 

 through parenchyma cells was immensely slower than 

 through sieve tubes and is more nearly of the order of 

 observed protoplasmic streaming in these cells, lends 

 weight to this interpretation. Schumacher suggests that 

 mere penetration into the protoplasm does not determine 

 movement but that some secondary, unknown factor, 

 Derhaps one concerned with polarity phenomena, deter- 

 mines movement. 



Schumacher is strongly opposed to the Miinch hypothesis 

 as to the mechanism of movement. He concludes that 

 protoplasm is the path of movement, but that the stream- 

 ing protoplasm is not concerned in the movement. As 

 has just been pointed out, however, it seems that the 

 motion of the protoplasm may be responsible or that 

 movement may take place by a mechanism like that 

 proposed by Van den Honert, which may be considered 

 as a special case of protoplasmic streaming. 



Although I am thoroughly convinced that Schumacher's 

 experiments and observations demonstrate the effective- 

 ness of the sieve tubes as transporting cells and that 

 materials may be carried by the same mechanism by which 

 the dye is carried, I hesitate to conclude without more 

 extensive evidence that the rate, direction, and mechanism 

 of movement through the sieve tubes of carbohydrates, 

 nitrogen compounds, and such solutes will be strictly 

 comparable to those of fluorescein. As is made clear in 

 Chaps. IV and V, the movement of solutes such as dyes 

 that have been introduced into xylem tissues has led to 

 false conclusions relative to the normal tissues concerned 

 in movement as well as to rates and direction of movement. 

 It is perhaps equally possible that fluorescein movement 

 through sieve tubes is also misleading. It is true the 

 movement of fluorescein through sieve tubes seems much 



