Spiraea Ulmaria, L., and its Bearing on the Problem 
of Xeromorphy in Marsh Plants. 
BY 
R. H. YAPP, M.A., 
Professor oj Botany in the University College of Wales , Aberystwyth. 
With Plates LXXXI-LXXXIII and eleven Figures in the Text. 
Contents. 
SECTIONS PAGE 
1. Introduction 815 
2. Hairiness in Marsh Plants 816 
3. Distribution of Certain Hairy. Species of Spiraea 816 
4. Habit and Habitats of Spiraea Ulmaria 817 
5. The Distribution of Pubescence on the Leaves 819 
6. Comparison with other Hairy Plants ,821 
7. Seasonal Differences in Leaf Structure 822 
8. Development of the Leaves, and Time of Origin of the Hairs, Palisade 
Cells, etc 830 
9. The Effect of Environment on the Production of Hairs and Palisade 
Cells 834 
10. The Localization of Withering in Leaves exposed to Wind, etc. . . 840 
11. The Causes of the Formation of Hairs and Palisade Cells . . . 842 
12. The Physiological Effect of the Hairs, etc. ... . . 850 
13. The Xeromorphy of Spiraea Ulmaria 852 
14. The Present Position of our Knowledge of ‘ Swamp Xerophytes and 
a Tentative Hypothesis 854 
Summary of Results 861 
Bibliography 866 
§ 1. Introduction. 
I N Warming’s ‘ Pflanzengeographie published in 1896, occurs the 
following passage 1 : ‘ Das Ideal der wissenschaftlichen Behandlung 
der einzelnen Vereine muss der wissenschaftliche Nachweis dafiir sein, wie 
jedes einzelne seiner Mitglieder (Lebensformen) im morphologischen, im 
anatomischen und im physiologischen Einklange mit den verschiedenen 
okonomischen und geselligen Verhaltnissen, worunter es lebt, ist.’ We are 
as yet far from the attainment of this ideal in respect of even a single plant- 
1 Warming (’96), p. 119. 
L'Annals of Botany, Vol. XXVI. No. CIII. July, 191s.] 
