Problem of Xeromorphy in Marsh Plants . 851 
Wiegand 1 has recently experimented with various materials, so arranged 
as to simulate the hairy and cutinized coverings of plants, and determined 
the extent to which they hindered evaporation. From his experiments he 
concluded that hairy coverings (even when of a thin, strigose nature) markedly 
retard transpiration in wind, but have comparatively little effect in still air. 
They also have a marked retarding effect in sunshine. Further, that a waxy 
(i. e. cutinized) covering is more efficient in reducing transpiration than 
a hairy one. 2 
Wiegand’s results confirm the previously expressed opinions of 
Goebel 3 and Haberlandt, 4 that a dense covering of hairs may be especially 
effective in reducing transpiration when the plant is exposed to strong winds. 
Haberlandt also refers to their similar action under strong insolation. On 
theoretical grounds it is to be expected that the hairs would have this effect. 
Thus, when the leaf is exposed to wind, the effect of the hairs would be to 
diminish the gradient of density of water vapour outside the leaf, by 
hindering the removal of the humid air from its surface. The distance 
between the absorbing surface (the free air outside) and the supply is thus 
increased, and the rate of flow of water vapour outwards correspondingly 
diminished. 5 On the other hand, if the air outside the leaf be perfectly still, 
the gradient of density would be much the same whether the hairs were 
present or not. Further, wind passing rapidly over the unprotected surface 
of a leaf would reduce the pressure just at the entrances of the stomatal 
apertures, and so tend to suck air (and consequently water vapour) out of 
them. 6 This effect also would be prevented by the hairs. Again, when 
exposed to direct insolation, white, air-containing hairs, such as those of 
Spiraea , must reflect an appreciable amount of light. In consequence, less 
heat is absorbed, and the transpiration again diminished. 
In the case of Spiraea Ulmaria , it has been shown that its characteristic 
pubescence is localized precisely in those parts of the plant which in nature 
are most exposed to the drying effects of wind, sun, &c. Conversely, the 
glabrous leaves are those found under conditions where transpiration and 
the effect of wind are at a minimum. The same is also true of the majority 
of plants mentioned in § 6. Perhaps the most striking instance of all is 
1 Wiegand (’10), p. 430 et seq. He used blotting-paper as the evaporating surface, laying 
over this various cotton materials to represent hairy coverings, and bees-wax for cutin. 
2 Probably Wiegand has exaggerated the differences between the two kinds of covering, as in his 
experiments the layer of wax would have nothing to represent stomata, while all the cotton fabrics 
would be abundantly furnished with small apertures. 
3 Goebel (’91), p. 18. 4 Haberlandt (’09), p. 116. 
5 Brown and Escombe (’00), footnote to p. 276, suggest that epidermal hairs may lessen the 
amount of C0 2 absorbed by the leaf when the air is in motion, by preventing air currents from 
sweeping away the external density 1 shells ’. The same must apply equally to the transpiration of 
water vapour. 
6 This action is similar, as Dr. Schott suggests, to that of a hair-dresser’s sprayer, or of a Gifford 
injector in sucking water into a boiler. 
3 K 2 
