Differentiation 221 



Studies of geotropic reaction of typical roots and stems show that they 

 are also different in their response to auxin, the growth of roots being 

 inhibited by concentrations which stimulate growth of stems, a fact which 

 explains the geotropic reactions characteristic of these two organs. They 

 differ physiologically in other respects, for Collander ( 1941 ) has shown 

 that certain cations may be differentially distributed between root and 

 shoot, sodium and manganese being more abundant in the former and 

 calcium, strontium, and lithium in the latter. 



Differences between vegetative and reproductive phases of the life 

 cycle are sometimes physiological as well as structural. Many early workers 

 (Sachs, 1880, 1882) noticed the difference between "blind" and "flower- 

 ing" stems, the former when used as cuttings producing vegetative growth 

 only and the latter, flowering shoots. This difference has now been shown 

 to be related to the presence of some substance or substances which 

 induce flowering (p. 397). Torrey (1953) reports that three synthetic 

 substances which inhibit root elongation have specific effects on the ac- 

 celeration or retardation of the differentiation of xylem and of phloem. 



Physiological differentiation must evidently be important in sex deter- 

 mination, and chemical differences between the sexes have been found 

 by several workers. By the Manoilov reaction, for example, staminate 

 and pistillate plants of poplar can be distinguished, as well as "plus" and 

 "minus" strains of Mucor ( Satina and Blakeslee, 1926 ) . Stanfield ( 1944 ) 

 has described chemical differences between staminate and pistillate 

 plants of Lychnis dioica. Aitchison (1953) found that in several genera 

 the sexes were unlike in oxidase activity, this being greater in some cases 

 in males and in others in females. Hoxmeier (1953), working with Canna- 

 bis and Spinacia, reports that the tissue fluids of staminate plants are more 

 acid than those of pistillate ones. In Cannabis, Cheuvart (1954) observed 

 differences between the sexes in chlorophyll content, especially in the rate 

 at which this is reduced at the time of flowering. Reinders-Gouwentak 

 and van der Veen ( 1953 ) found that in poplar the female catkins tended 

 to stimulate wood formation on the stem below them whereas males did 

 not, suggesting a difference between the sexes here in the production of 

 a growth substance. 



Regular changes in the physiological activity of the series of successive 

 leaves on a plant, related to both position and age, have been observed 

 by various workers. Dormer ( 1951 ) determined the dry weight per unit 

 of length in successive internodes of Vicia from the apex downward and 

 found that during the unfolding of the ninth leaf there was a sudden 

 change in the distribution of the dry-weight increment. The nutritional 

 history of an internode thus seems to be a function of its position in the 

 stem. The developing seedling has also been shown to change in its physi- 



