Various Physical Factors 361 



tern which has arisen through natural selection, as have other embryonic 



characters. 



The relation between size and form here seems to be too immediate, 

 however, to be accounted for in this long-range fashion. An inherited 

 pattern ought to be evident in a small and stunted as well as in a large 

 individual, but the effect of size seems to be more direct. One is tempted 

 to see here another example of the regular spacing of structures to which 

 attention has earlier been called (p. 199). In the same way there may be 

 maintained a constant ratio between primary xylem and phloem, a ratio 

 which originates within the tissues of the primary meristem. This morpho- 

 genetic problem, which has an important bearing on the origin of differ- 

 entiation and of structure in general, seems especially favorable for 

 biochemical and perhaps even mathematical attack. 



Bioelectrical Factors. For many years the possibility that electricity, 

 in one way or another, might affect the physiological and developmental 

 activities of plants has interested botanists, and there is a very consider- 

 able literature on the subject. Unfortunately, this is a field that is theo- 

 retically and technically so complex that few workers are qualified to 

 obtain dependable results in it, and much of the published work is there- 

 fore of doubtful value. The present writer is certainly not competent to 

 review it critically. 



Students of tropisms have discussed the possibility that differences in 

 electrical potential may be involved in these activities. Went (1932) 

 suggested that the polar flow of auxin is an electrophoretic process. 

 Clark (1938) raised doubt as to this idea and pointed out that it is pos- 

 sible by certain chemical treatments to abolish the polarity of auxin trans- 

 port without changing the electrical polarity, protoplasmic streaming, or 

 other characters of the system (p. 385). Schrank, however, who has dis- 

 cussed the problem in some detail ( 1957 ) , has shown that in the trans- 

 verse distribution of auxin in the Avena coleoptile, a transverse electrical 

 polarity precedes the movement of auxin, thus tending to support Went's 

 theory. The early work on the electrical control of polarity was reviewed 

 by Thomas (1939). 



Morphogenetically the most significant result of the work with elec- 

 tricity is the evidence that in many organisms there are continuous bio- 

 electrical currents and a distribution of electrical potential differences 

 so constant that a bioelectrical field is set up in the organism. By the use 

 of a very delicate micropotentiometer Burr was able to demonstrate 

 the existence of such a field in amphibian eggs (1941). Even in the 

 young ovaries of cucurbits such relations between the form of the ovary 

 and the bioelectrical pattern could be shown (1944). Burr and Northrop 

 ( 1939 ) have discussed the general problem of electrodynamic fields in 

 living organisms. Lund ( 1931 and others ) carried out a series of studies on 



