Problem of Xeromorphy in Marsh Plants. 863 
investigators show that in general these are most fully developed under 
conditions which impose on the plant some necessity for increased economy 
of water, i. e. under conditions which either promote transpiration or 
hinder absorption. Spiraea Ulmaria is found to be less plastic than many 
other species in respect to hairiness. Many experiments show that it is 
difficult to inhibit the formation of hairs on leaves which are normally 
hairy, though their number and length can be readily influenced. At 
the same time, the distribution of pubescence on the plant (described in 
2 above) corresponds in the most striking manner with the average 
physical conditions of the natural environment. Thus the curves of hairiness 
in Spiraea follow very closely the mean curves of evaporation and light- 
intensity. This is true whether the vertical changes in these factors (due to 
the density of the vegetation) or the annual march of evaporation and light- 
intensity are considered (cf. PI. LXXXI and Text-figs. 9 and 10). 
7. The localization of hairs on the partially hairy leaves of Spiraea 
(in particular the marginal and interveinal bands of hairs) shows a marked 
correspondence with the localization of withering in leaves in general, when 
such withering is due to the effect of wind, &c. (cf. PI. LXXXII and 
LXXXIII, and Text-fig. 11). Anthocyan in young leaves in spring-time, 
the local effect of freezing, &c., often show a similar distribution. As 
regards their water supply, these regions of the leaf-lamina (i. e. the margins 
and interveinal parts) are physiologically different to the remaining parts. 
They are more liable to lose water, and are also the most remote from 
the water supply coming up from below. 
8. The development of the leaves is described, and it is shown that no 
hairs are developed in the bud until the first foliage leaves have expanded 
and become functional. Hairs then appear on the main veins of the 
developing leaves, and the hairiness of these veins (while still in the bud) 
increases pari passu with the general increase of transpiration from the 
shoot. But the hairs on the finer veins, and those which occur over the 
parenchyma ‘ islands only appear when the leaf has emerged from the 
bud ; i. e. when some parts of it are already in contact with the outer air 
(Text-figs. 7 and 8). It is these later formed hairs which are most 
conspicuous in the mature leaves (see 2 above). 
9. The causes of the formation of the hairs, &c., are discussed, and it is 
shown that a parallel may be drawn between the epidermal hairs of leaves 
and stems, the palisade cells of leaves, and to some extent root-hairs as 
well. In so far as the development of these special elongated elements 
is determined or accelerated by external conditions, it would appear that 
similar factors are usually operative in all three cases. 
Hair formation in Spiraea Ulmaria is considered in detail. In this 
case the factors which determine the presence or absence of hairs on the 
leaves are probably the following : (a) heredity, (b) individual variation, 
