Problem of Xeromorphy in Marsh Plants. 835 
Eberhardt 1 showed that plants grown in dry air tend to be more hairy, 
and their leaves thicker (in consequence of the great development of pali- 
sade), than those grown either in normal or in very humid air. 
Zinger , 2 in an interesting paper on some European species of Came Una , 
shows that in C. microcarpa and the different varieties of C. saliva , a gradual 
transition from a xerophytic to a hygrophytic type can be traced, with 
marked differences in respect of palisade tissue, hairiness, &c. These differ- 
ences are strictly correlated with the conditions of illumination and moisture 
in the respective natural habitats. 
Lesage 3 found that when plants were grown in a saline soil, or solutions 
in which the salts reached a certain concentration, they developed more 
palisade than when grown in ordinary soil or very dilute salt solutions. 
Later 4 he subjected growing plants to periodic variations of atmospheric 
pressure. Those which had been exposed to the periodic lowering of pres- 
sure had (if leaves of the same age, &c., were compared) more marked 
palisade, than the control plants grown under ordinary air pressure. 
Similar results to those of the above-mentioned authors have been 
obtained by other experimenters . 5 
Although the question of root-hairs scarcely comes within the scope of 
this paper, it may be mentioned that their formation also is largely depen- 
dent on the water supply. Schwarz 6 found that no root-hairs were formed 
in very wet soil. Further, that as the water content of the soil decreased, 
root-hairs increased correspondingly. Finally, however, when the soil 
attained a certain degree of dryness, root-hairs were again suppressed. 
If the above results be analysed, it will be noticed 7 that in general , 8 
both palisade cells and the epidermal hairs of aerial shoots (and to some 
extent at least , root- hair s') are most fully developed under conditions which 
either promote transpiration or hinder absorption ; namely, under con- 
ditions which impose on the plant some necessity for increased economy 
of water. 
The results of the writer's own work on Spiraea Ulmaria and other 
marsh plants may now be given. 
(b) Field observations on Spiraea , &c. It has been stated above that 
even the type-form of Spiraea Ulmaria produces both glabrous and hairy 
1 Eberhardt (’03), pp. 149-52. 2 Zinger (’09), pp. 340-9. 
3 Lesage (’89), p. 204, and (’90), p. 170. Cf. also Boodle (’04, second paper), p. 41. 
4 Lesage (’94), p. 255. 
5 e. g. G^neau de Lamarliere (’92), p. 483 et seq., Lothelier (’93), p. 520, &c. Cf. also 
Burgerstein (’04), pp. 47-53, and the additional literature cited by him. 
6 Schwarz (’83), p. 14 et seq. Cf. also Sorauer (’86), pp. 95-6, and Mer (’83), p. 117. 
7 Cf. Lesage (’94), and Burgerstein (’04), pp. 51 and 53. 
8 Of course plants vary much in plasticity. Stahl (’83), pp. 5-10, instances cases of pronounced 
shade and sun plants which have little or no power of adapting themselves to sunny and shady 
habitats respectively. 
3 1 2 
