SUSCEPTIBILITY GRADIENTS. 85 



this kind is made up of hairs of very different stages of develop- 

 ment and physiological age, the most basal hairs being in general 

 the earliest developed and so the largest and the oldest, while 

 the most apical are the latest, smallest and youngest, with inter- 

 mediate stages between the two extremes. The differences in 

 physiological age in such a group are beautifully shown by the 

 differences in susceptibility, the youngest hairs being most sus- 

 ceptible, the oldest least, with a general gradation in suscepti- 

 bility between. These age differences appear even in the sus- 

 ceptibility of the apical cells, those cells of the young hair being 

 somewhat more susceptible than those of the old, but this dif- 

 ference between apical cells of young and old hairs is much less 

 than that between the more basal cells. In KCN m/iooo for 

 example, apical cells of the youngest hairs die almost at once, 

 those of the oldest hairs after 1-5 minutes, but in the youngest 

 hairs death is complete all the way to the basal cell in 10-15 

 minutes while in the oldest the most basal cells may live for 2-3 

 hours. In the young hair the basal cells have not yet had time 

 to become old, while in the old hair the basal cells have reached 

 an advanced stage of physiological senescence, and in the apical 

 cells senescence has progressed to a greater or less extent, but 

 has been counterbalanced to some extent by the periodic re- 

 juvenescence accompanying cell reproduction (Child, '150, 

 Chap. X.). These susceptibility relations between the older 

 and younger hairs of a single group and between apical and 

 basal regions of a single hair constitute a most beautiful and 

 striking example of the course of senescence in plant axes. 



The attempts to reverse the whole hair gradient were made 

 with tufts of this sort, consisting often of hundreds of hairs of all 

 ages and stages of development, their attachment to the branch 

 from which they arose being of course undisturbed. In this way 

 it was possible to observe the difference in the reaction of old 

 and young hairs to particular experimental conditions. 



The result of a few experiments along this line which serve 

 merely to suggest the possibilities, are briefly stated. Exposure 

 to temperatures of 3O-35 C. for three or four hours killed the 

 youngest hairs completely or all but one or two basal cells, almost 

 without exception. The hairs of medium age and susceptibility, 



