248 TRANSLOCATION IN PLANTS 



concentration and low turgor pressure. Those meri- 

 stematic cells which are more active in removing solutes 

 by respiration or by deposition as osmotically inactive 

 protoplasm or as cell wall material, will tend to bring 

 more solutes to themselves than will less active cells which 

 may receive nothing if their turgor remains high. Storage 

 cells which can remove osmotically active sugar, by 

 deposition of starch, for example, will continue to receive 

 a supply from the phloem so long as they are able to 

 maintain a relatively low turgor pressure. Theoretically 

 the mechanism seems well adapted to account for directive 

 flow to tissues capable of removing solutes. The main 

 obstacle to this interpretation, however, is the fact that 

 the receiving cell frequently has a higher osmotic concen- 

 tration or a high turgor, or both than the supplying cells 

 (see 3a of Sec. 30). A directive flow of specific substances 

 also cannot be accounted for either according to the Miinch 

 hypothesis or by the modification suggested by Crafts. 

 Other weaknesses of the proposed mechanism have already 

 been discussed in Sec. 30. 



That distribution of materials, whether brought about 

 by diffusion, by protoplasmic streaming, by spread in an 

 interfacial film, or by pressure gradients, may be a factor 

 in phenomena of dominance and regeneration is probable, 

 but it also seems probable that it is perhaps only one of 

 several factors. For example, the effect of gravity on the 

 distribution of materials and on dominance is not easily 

 explainable on the basis of rate or direction of movement 

 as influenced by diffusion gradients, rates of streaming, or 

 pressure gradients alone. 



Reed and Halma (1919a) have reported a marked 

 influence of gravity on dominance of buds on stems of the 

 Chinese lemon. In horizontal stems apical dominance 

 was greatly reduced and buds along the upper side of the 

 stem developed shoots. If the stem were then turned 

 over inverting the sprouts, these ceased growing and died 

 as shoots from buds on the upper side replaced them. 

 This may be partly explained on the basis that gravity 



