IRRITABILITY; POLARITY AND CORRELATION 355 



Among the qualitative correlations is that between the main 

 and lateral axes of Gymnosperms (Chap. XVII). Another equally 

 interesting example is seen in the potato which produces normally 

 two kinds of stems: the aerial, leafy ones and the subterranean 

 ones which bear the tubers. If the aerial stems are cut away, 

 one of the subterranean stems will turn up into the air and de- 

 velop leaves and branches. If all the underground buds are re- 

 moved so that no stems are present below ground, tubers will 

 form on the lower part of the stem above ground. These tubers 

 are similar to underground ones, but are bright red in color and 

 bear leaves at the eyes. If the top of the stem is darkened, the 

 tubers will form at the apex, showing that the translocation of 

 food is here up instead of down, in which case the polarity has 

 also been reversed to a certain extent. Such experiments serve 

 to show the close connection between correlation and polarity, 

 but these problems are all very complex and are tied up with the 

 whole question of regeneration and growth, upon which much 

 work remains to be done. 



The relation between polarity and correlations is well illus- 

 trated by the recent work of Lund, who has found that the cells 

 of onion roots, Bryophyllum leaves, and Douglas fir shoots generate 

 electric currents continuously. The electric polarity of each cell 

 is oriented in such a fashion that the cells act like a series of bat- 

 teries. Their respective individual E.M.F.'s are added and thus 

 may build up a considerable difference in potential. In some cases, 

 however, the currents developed may oppose and neutralize each 

 other. In the Douglas fir, the apex of the stem acts as a positive 

 pole in respect to the base of the stem and the apex of the lateral 

 branches. This electrical dominance of the main stem seems to 

 be correlated with the physiological dominance of the main axis; 

 and Lund believes that electric currents flow constantly along 

 certain circuit channels of the tree and thus unify and correlate 

 the living regions into a definite electrical system. 



QUESTIONS AND PROBLEMS 



1. Of what " survival value" is the positive geotropism of the root system? 



2. If a weight of one pound is held in the left hand and it requires a weight 

 of 1 lb. 2 oz. in the right before an increase can be detected, what must the 

 weight in the right hand be if the weight in the left is 3 lb. 8 oz.? 



3. Gravity is attracting a root with the force X and moist soil at one side 



