88 PATTERNS AND PROBLEMS OF DEVELOPMENT 



gresses basipetally from its tip (Fig. 26, A, B). Only in somewhat later 

 stages, when the developing thallus consists of several or many cells, does 

 its apical region become more susceptible than other parts. From these 

 stages on, cytolysis progresses basipetally from, the tip of the thallus and 

 from the tip of each rhizoid present (Fig. 26, C). Reduction of potassium 

 permanganate shows the same gradient pattern (Child, 1919a). An elec- 

 tric polarity is present in single cells of Pithophora and Nitella (Lund, 

 1938). Mold hyphae developing from spores show a basipetal suscepti- 

 bility gradient. 



Axial differences in rate of respiration in the higher plants are often of 

 uncertain significance because the proportions of active and relatively 

 inactive or dead cells and of protoplasm and water, cellulose or other non- 

 living substances differ at different levels. Structural differences along the 

 axis of the potato tuber are not great before sprouting occurs: basal 

 halves produce from 6.5 to 6.8 per cent less carbon dioxide per unit of 

 weight than apical halves in a variety tested; after apical sprouts begin 

 to grow, the difference is 41.8 per cent, and after removal of sprouts, 10.7 

 to 19.8 per cent (Appleman, 191 5). The fact that apical "eyes" tend to 

 develop first and inhibit others more or less completely suggests that there 

 is an axiate pattern in the quiescent tuber and that the difference in CO2 

 production is a feature of it. 



Many observations by many investigators have shown that electric po- 

 larity is a characteristic feature of plant axes, and various sources of origin 

 of the potential differences observed have been suggested. According to 

 Lund and his students, the growing tip of the onion root and of other 

 roots and the tips of the main stem and branches of the Douglas fir are 

 externally electropositive to lower levels of the same axis, and the tip of 

 the main axis of the fir is positive to the tips of lateral branches of the first 

 whorl.' Rate of methylene blue reduction and CO2 production in the root 

 are highest in the most positive region and decrease basipetally with it to 

 a level some distance from the tip, where there is a region showing some 

 increase, followed again by decrease. In both root and fir stimulation of a 

 tip decreases its positivity and may reverse the potential gradient. The 

 electric polarity of the fir shows a differential susceptibility to tempera- 

 ture. Decrease in temperature decreases electric polarity because decrease 

 of positivity is greater in regions of high than of low positivity; in other 

 words, a differential inhibition apparently occurs. With return to the 



■ Lund, 1928a, b, c, 1929a, b, 1930, 1931a, b, c, 1932a, b; Lund and Kenyon, 1927; Marsh, 

 1928. 



