Doyle — Some Researches in Experimental Morphology. 433 



alone; but if tlie stimulus of tension is also present, there is a still further 

 marked increase. Tims, -without explaining his metliods, he measured tendrils 

 which had remained free, tendrils stimulated by contact alone, and tendrils 

 stimulated by both contact and tension. The average breaking-strengths of 

 the three were : — 



Free tendril, 190 grms. 



Tendril subject to contact only, , . . 651 ,, 

 Tendril subject to contact and tension, . 1007 „ 



Again, by other methods, he measured the results with tendrils subject to 

 tension only. 



Breaking strength of tendril under no tension, . 862 grms. 

 ,, ,, ,, with tension, . . 1239 ,, 



This result, coupled with Bonlner's, seems to indicate A^ery positively, 

 indeed, that tension can have very marked effect on tissue. And so it seems 

 probable that the increase of mechanical strain has played a very important 

 r61e in the development of the secondary xylem. -There is one fact which, I 

 think, speaks in favour of it. Unfortunately it was not at first considered 

 that mechanical effects would be of any large direct importance, and so it was 

 only at the very last moment that it was thought fit to make such measure- 

 ments as those below. Tlie material necessary, which might otlierwise have 

 been preserved, was then found to have been in most cases sacrificed- 

 However, the following was accurately done. The area of the wood in the 

 petiole Sol. Balbesii, A, 13119, 24/5-26/1/14, was carefully drawn with the 

 Abbe camera lucida at a maguificatiuu of 40 diams. The area of wood in 

 the stem below was similarly drawn, and the two areas compared by weighing, 

 with the remarkable result : — 



Area of wood in petiole. . . . 24'2 sq. mms. 

 ,, „ ,, stem below, . . 16'4 „ „ 



Further in the stem below there were two rings of wood quite distinctly 

 to be seen, indeed nearly every section was split at parts along the line 

 between the two rings, If, as seems to be plausible, the two rings are 

 explained by the fact that the vegetative portion had been completely 

 removed from the plant at the time of grafting, that thus the plant rested for 

 a fairly prolonged period without any marked foliage development, and that 

 then with the active growth of the small scion once the union was firmly 

 established, a second vegetative period for the plant began just comparable 

 to the appearance of the Johannis branch in the ash ;— if this is so, the outer 

 ring only in the stem corresponds to the wood-development in the petiole. 



