844 Yapp. — Spiraea Ulmaria , L., and its Bearing on the 
elements are developed especially under conditions which either promote 
transpiration or hinder absorption (see p. 835). For S. Ulmaria , &c , in 
particular, we have seen that in nature the curves of hairiness follow very 
closely the mean curves of evaporation, &c. It is obvious that such 
external conditions can only act by inducing internal changes in the cells 
themselves. Kohl ( 1 . c.) has already suggested that changes of turgor may 
be important in this connexion. 
2. What may be termed topographical evidence. It has been shown 
(§§ 5 ( d ) and 10) that under certain conditions the margins of leaves and 
the thinner areas between the main veins exhibit phenomena which the 
other parts of the leaf do not. Also that these parts are not only especially 
liable to lose water, but are situated furthest from the water supply coming 
up from below. Thus it is easy to understand that any shortage in the 
supply of water to the leaf will affect these regions first and most severely. 
That such is the case is evidenced by the localization of withering from 
exposure to wind. 1 These considerations suggest that the marked localiza- 
tion of hairs (and to some extent palisade cells, see p. 830) in the same 
regions of the leaf may also be connected with a diminished water supply. 
This is, of course, especially so in view of the known effects of certain external 
factors (see 1 above). 
3. Evidence derived from leaf -development in Spiraea. As regards the 
hairs, we must distinguish between those formed in the bud and those 
formed during the unfolding of the leaf (see § 8). 
It has been shown that, while the earliest leaves are entirely glabrous, 
the later ones become progressively more hairy, the hairs on the -larger 
veins being formed while the leaf is still in the bud (see Text-figs. 7 and 8). 
This increasing hairiness of the larger veins of the developing leaves occurs 
pari passu with the general increase of transpiration from the shoot. The 
latter is of course due partly to the increase of transpiring surface as more 
leaves are unfolded, and partly to the march of the seasons. Knowing the 
effect in general of a dry habitat on hairiness, it is difficult to avoid the 
suggestion that this increase of hairiness may be connected with a diminu- 
tion of the water supply to the apical bud. This may readily be brought 
about by a partial deflexion of the water current into the continually 
increasing number of mature leaves. In this way the cells of the young 
developing leaves will find it increasingly difficult to maintain themselves 
in a complete and constant state of turgor. If this be so, it may be 
regarded as another case of what Wiesner 2 has called £ correlative transpira- 
tion \ Under this term he includes all cases in which strongly transpiring 
organs deprive others (which transpire little or not at all) of water. Such 
displacements of water not infrequently bring about striking results. Thus 
1 Hansen (’ 04 ), p. 43, &c., discusses the withering of leaves as a result of withdrawal of water 
by wind, 2 Wiesner (’ 05 ), p. 477 et seq. 
