840 Yapp . — Spiraea Ulmaria , A., and its Bearing on the 
The var. denudata , as well as several intermediate forms, has been 
grown for some years. The results show that the individual, in its succes- 
sive generations of shoots, retains to a considerable extent its peculiar 
characteristics, even under varying habitat conditions. For example, 
5 . denudata has never been induced to become really hairy, even when 
grown in dry, sunny, exposed situations. 
The results recorded in this section show that (1) the production 
of glabrous and hairy leaves in Spiraea Ulmaria and certain other marsh 
plants closely follows in nature the mean curves of evaporation and light- 
intensity ; but that (2) 5 . Ulmaria possesses a more limited plasticity, 
in respect to hairiness, than that exhibited by such species as Mentha 
aquatic a , &c. 
§ 10. The Localization of Withering in Leaves exposed to 
Wind, etc. 
Before discussing the causes of hair formation, certain parallel pheno- 
mena must be mentioned. As stated above (p. 820), the partially hairy 
leaves and leaflets of Spiraea exhibit a definite localization of the hairs (cf. 
PI. LXXXII and LXXXIII). It was early noticed 1 that when Spiraea 
plants with glabrous leaves were exposed to winds sufficiently strong to 
cause partial withering, the distribution of the withered parts coincided very 
nearly with the distribution of hairs on the partly hairy leaves of the same 
species. Exposure withering thus begins round the margins of the leaflet, and 
sometimes, though not always, extends inwards in the form of bands between 
the main veins. Subsequently observations were made on the localization 
of withering in the leaves of many other plants. These, which for the most 
part confirm the results of Schroder , 2 published in 1909, may now be sum- 
marized (cf. Text-fig. 11). 
Leaves which are allowed to wither naturally in still air lose their 
colour gradually, and either more or less uniformly or else irregularly. On 
the other hand, the effect of exposure to strong winds, & c., is to cause first 
a marked withering of projecting apices, e. g. the apex in linear leaves , 3 &c., 
leaf-teeth, or the various tips of lobed leaves (see Text-fig. 11, A, B, and E). 
Next, in other than linear leaves, the withering extends round the margin 
of the leaf, and sometimes, though by no means always (even the same 
species may vary in this respect, cf. Text-fig. 11, C and d), between the 
larger veins (Text-fig. 11, b-g). 4 The process may now stop, or it may 
1 Yapp (’08), p. 691. 
2 Schroder (’09), pp. 85, &c. Apart from this paper the literature on the withering of leaves is 
scanty. Hansen (’01), p. 32, and (’04), p. 34, calls attention to the withering of leaf apices and 
margins through wind. Hasenclever (’79), and Oliver (’93), p. 22, show that acid gases will cause 
discoloration of the apices and margins of leaves. Cf. also Stahl (’09), pp. 134-5* 
3 Cf. Yapp (’08, second paper), p. 65. 
4 My attention was called by Professor F. W. Oliver to the exceptional case of Polygonum 
