850 Yapp . — Spiraea Ulmaria , Z„, and its Bearing on the 
decreased turgor appears to afford to the hair-producing cells is probably more 
of the nature of an accelerating stimulus than an actually determining one. 
Further, different parts of the plant vary in plasticity. As mentioned 
above, the lowest leaves are not plastic, but are persistently glabrous. The 
upper leaves on the erect flowering shoots, on the other hand, are fairly 
persistently hairy. The most plastic leaves of all appear to be those 
occupying an intermediate position, i. e. the lower hairy ones. Many plants 
are more plastic than Spiraea in regard to hairiness ; but in all cases 
hairiness appears to be associated with relatively dry, and glabrousness with 
relatively humid conditions. Taking these facts into account, it would seem 
that Spiraea Ulmaria is retaining a certain degree of plasticity in regard to 
hairiness. But at the same time it is tending to become fixed in the 
direction of response to the average conditions, to which the upper and 
lower leaves of the species, respectively, have been exposed in nature for 
many generations. 
§ 12. The Physiological Effect of the Hairs, etc. 
Most authors agree that a dense covering of dead, cutinized hairs 
depresses transpiration. This is said to be due to the retarding of gaseous 
interchange on the transpiring surface, and also, in some cases, to the hairs 
acting as a screen against intense insolation. According to the distribution 
of the hairs on the respective surfaces of the leaf, one or other effect will be 
more marked. But, as Haberlandt 1 has pointed out, there is an absence of 
critical researches on the subject. At first sight, Spiraea Ulmaria , with its 
glabrous and hairy leaves, would seem to be admirably adapted for such 
critical researches. But, as we have seen, the progress towards hairiness 
in the leaves of this species is accompanied by other structural changes. 
These also cannot but exert an influence on transpiration. The whole 
question of the relation between leaf structure and transpiration is in 
a somewhat chaotic condition, different experimenters having obtained 
diametrically opposed results. Thus, von Hoenel 2 found that under similar 
conditions ‘ shade leaves 5 transpired more strongly than * sun leaves \ 
Geneau de Lamarliere , 3 * on the other hand, obtained exactly opposite results. 
The question is a complicated one, and the author hopes to deal with it more 
fully and critically in a future paper. For this reason no attempt will be 
made at the present time to do more than put forward certain a priori 
considerations, and to refer briefly to the work of other botanists. 
1 Haberlandt (’ 09 ), p. 116. This author himself performed some experiments on the effect of 
hairs on transpiration (1. c., p. 138). 
2 Von Hoenel (’ 79 ), quoted by Burgerstein (’ 04 ). 
3 Geneau de Lamarliere (’ 92 ), p. 542. Cf. also the literature cited in Burgerstein (’ 04 ), p. 55 
et seq. Also Ilesselman (’ 04 ), p. 402 et seq. 
