Problem of Xeromorphy in Marsh Plants. 845 
Wiesner ( 1 . c.) observed the production of anisophylly in Aesculus under the 
influence of correlative transpiration. Again, Schimper, 1 Haberlandt, 2 and 
others found that the lower leaves of certain plants yielded up their water 
to the younger leaves when subjected to drought. Two cases which approach 
that of Spiraea very nearly are recorded by Smith and Balls. Smith 3 pro- 
duced evidence to show that during the day-time transpiration from adult 
bamboo culms may be so great as to draw off the supply of water from the 
growing culms, thus slowing down their rate of growth. Similarly, Balls 4 
found that in summer-time in Egypt the growth of the stem of cotton plants 
and others (e. g. Helianthus ) was completely arrested whenever direct sun 
was shining on the plant. This he showed to be due to the consumption of 
all the available water in transpiration from the already unfolded leaves. 
The water supply was thus the limiting factor in stem growth. These 
instances add to the probability that a similar disturbance of the supply of 
water to the apical bud takes place in Spiraea Ulmaria , and that this 
disturbance affects the production of hairs in the bud. 
But most of the leaf hairs, as well as the palisade cells, actually elon- 
gate after the leaf emerges from the bud, and while it is unfolding in direct 
contact with the outer air (p. 833). At this stage cuticular transpiration 
must be an important factor. 5 Owing to its vernation, the parts directly 
exposed to atmospheric influences when the leaf first emerges from the 
bud are the larger veins (which are already covered with hairs) and the 
delicate leaf margins (cf. Text- figs. 8 and 2, leaf 6). Naturally, therefore, 
the loss of water by cuticular transpiration will be most severely felt by the 
latter. As the leaf still further unfolds, and if the transpiration is sufficient, 
the next parts to experience a shortage of water will probably be the inter- 
veinal areas, because it is here less easily replenished. It is precisely in these 
regions that the hairs are localized in the partly hairy leaves. Finally, 
when the water supply is still further diminished, the lower surfaces of 
the unfolding leaves become completely covered with a dense pubescence. 
Thus the formation of both the hairs on the larger veins, as well as 
those on the smaller veins and other parts of the lamina, may be correlated 
with a diminished water supply to the developing leaves. In the former it 
seems probable that correlative transpiration plays the most important part. 
In the latter, loss of water by cuticular transpiration comes into play, 
coupled with (as before) a strictly limited supply of water from below. 
Further, the fact that all the cells of the upper leaves are much smaller 
than those of the lower (cf. Text-fig. 4) suggests development under con- 
ditions in which there is a partial deficiency of water. In other words, it is 
1 Schimpqr (’88), pp. 37-8. 2 Haberlandt (’09), p. 368. 
3 Smith, A. M. (’06), p. 325. 4 Balls (’10), p. 6, and (’ll), pp. 229-30. 
5 F. von Hoenel (’78) showed that very young developing leaves actually transpire more than 
mature ones. This he attributed entirely to cuticular transpiration. 
