THE MOLECULAR FORCES IN PLANTS. 187 



are left in water for some time, their surface becomes wet, and 

 then also the silvery lustre disappears. 1 When the leaves are 

 thus left for a long time under water, the water can undoubtedly 

 penetrate into the plant through the cuticle (but also in other 

 ways). If weighed leaves, in absence of light, are dipped with the 

 blade in water, while the cut surface of the leaf-stalk, which may 

 suitably be cemented with wax, remains unwetted, it is in fact 

 found that the leaves, taken after a time out of the water and 

 very carefully dried with blotting-paper, now weigh more than at 

 the beginning of the experiment. Naturally, this can only take 

 place when we experiment with leaves whose cells do not before- 

 hand exhibit their maximum turgescence. I obtained specially 

 good results when I dipped leaf-blades of Coffea arabica or 

 Syringa vulgaris in water for a shorter (three hours) or longer 

 (twenty hours) time, and when the leaves had been kept in a 

 shady place for two hours before making the experiment, so that 

 they were not too rich in water. 2 



1 See Sachs, Handbuch d. Experimentalphysiologie d. PJianzen, p. 159. 



2 See Detmer in Wollny's Forschungen, Bd. 1, Heft 2, and Journal f. Land- 

 wirthschaft, 27. Jahrgang, p. 105. 



71. Some Movements in Plant Structures Related to Their 

 Absorption of Water. 



The inner leaves of the involucre of Carlina acaulis, a plant 

 which grows on dry calcareous soils, show particularly interesting 

 phenomena due to absorption of water. If the whole inflorescence 

 is moistened, all the inner leaves of the involucre lay themselves 

 together (see Fig. 67) ; drying causes them to spread out again (see 

 Fig. 68). The involucral leaves possess a silvery white colour, 

 except along the middle of the underside, where they are coloured 

 brown. If a single involucral leaf be removed from the inflores- 

 cence of Carlina, and this brown region be moistened with water, 

 a movement immediately takes place, and the under side of the 

 leaf rapidly becomes convex in form. We prepare delicate trans- 

 verse sections through the middle part of an involucral leaf, and 

 note the following details of structure. An epidermis is present 

 on the uppsr and lower surfaces. The epidermal cells of the under 

 side are coloured brown. The bulk of the tissue enclosed by the 

 epidermis is composed of parenchyma, which is traversed by a few 



