848 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
established, might result in at least a temporary increase in the supply of 
water to the apical bud. This, by preventing the stimulus of decreased 
turgor, might perhaps bring about the formation of glabrous leaves. The 
results were not uniform, but this was to be expected, as we are dealing with 
a complex of variable factors. In each experiment, however, one or more 
glabrous leaves were sooner or later produced high up on an erect flowering 
shoot, where normally only hairy leaves are found. 
One example may be given. A normal shoot was selected, which on 
May 21, 1907, possessed seven unfolded leaves. Of these, leaves 1-3 were 
glabrous, leaves 4 and 5 partly hairy, and leaves 6 and 7 completely hairy. 
These seven leaves were then removed from the plant. At this date leaf 8 
(a hairy one) was unfolding. Subsequently, leaves 9 and 10 showed 
decreasing hairiness, while leaf 11 was quite glabrous. Leaf 12 had marginal 
hairs, leaves 13 and 14 exhibited increasing hairiness, while leaves 15-17 
(the last produced on this shoot) were hairy, though not densely so. 
It is difficult to say exactly what the reason is for this abnormal pro- 
duction of glabrous leaves. It may be that it is partly due to nutritive 
difficulties, for cutting off the leaves removes the assimilating as well as the 
transpiring surface. But if so, one would scarcely expect the return of 
hairiness described above. Taken in conjunction with the evidence 
previously given, the experiments, so far as they go, certainly suggest that 
the explanation may be, in part at least, the one already offered. 1 Some 
additional support of this view (i. e. that cutting off the transpiring surface 
may increase the supply of water to the apical bud) is furnished by an 
experiment of Balls’s. 2 This authors observations on the cessation of growth 
of the cotton plant during sunshine have been already cited. He further 
found that removal of about one-third of the total leaf area in full sunshine 
caused the terminal shoot to resume growth at once, owing apparently to 
the decreased loss of water from the plant. 
The causes which determine the presence or absence of hairs in Spiraea 
Ulmaria may now be summarized as a whole. Several distinct factors 
appear to be involved, i. e. : 
(a) Heredity. It is difficult to assign exact limits to the part played 
by this. The inherent tendency to form hairs in the first instance is certainly 
hereditary. But how far the degree of hairiness in var. denudata and the 
1 Although the experiments were undertaken more or less with the expectation of arriving at 
the results actually obtained, there is reason for caution in interpreting these results. On several 
occasions I found plants in the field, the stems of which had been injured an inch or two above the 
ground, apparently at an early age. In each case two or three of the cauline leaves immediately 
above the injury were less hairy than the normal, and in some instances even glabrous. Thus it 
would seem possible that the abnormal production of glabrous leaves described in the text is in some 
way connected with the injury caused by the removal of the leaves, rather than with the water supply 
as suggested. 2 Balls (TO), p. 9. 
