i8 93 . RAINFALL AND THE FORMS OF LEA VES. 97 



by the leaf is, as already noted, the lightening of the weight, the aid 

 to transpiration, and, lastly, the continual and thorough cleansing of 

 the surface from insects, fungi, etc. We can verify this on our home 

 plants. After a shower, the pointed leaves of ash, willow, etc., have 

 had the dust quite washed off, while rounded leaves like those of the 

 oak are still dirty. This cleansing of the leaves is of immense impor- 

 tance to tropical plants, as all will allow who have seen how they 

 may become overgrown with hepatics, algae, mosses, and lichens. 

 These epiphytes, which have many contrivances to enable them to 

 germinate and grow on the leaf, seriously interfere with its function, 

 and we are justified in looking for arrangements by which the leaf 

 will make it difficult for the epiphytes to get a footing. In the end 

 they are generally successful, however, and the leaves with longest 

 lives have the heaviest load to bear. 



Plants with shining satin-like leaves, often tinged on the under- 

 side with red, form a great group much cultivated for the sake of the 

 beautiful foliage. Anthurium crystalliimm, P telodendron Lindeni, with 

 begonias and many others, belong to this group. The peculiar 

 brilliancy of the leaves is due to papillose outgrowths of the epidermal 

 cells. These exert a capillary attraction on water, and the drops left 

 after a rainfall are thus dispersed by the papilla? into a thin sheath 

 which evaporates quickly and secures the rapid drying up of the leaf. 

 This capillarity was tested by dipping the tip of a leaf of A nthurium 

 in water, when the moisture was drawn up on the leaf 140 millimetres. 



The long points to the leaves are certainly, in countries like West 

 Java with a heavy rainfall, the chief means by which the plant dis- 

 perses the excessive water-supply. In China and Japan, where the 

 same conditions of humidity and great heat occur, they are a striking 

 feature of the vegetation. An interesting exception is found in plants 

 native to the drier regions along the shore or up on the mountains 

 which have rounded or crenate leaves. In our own woods, while we 

 occasionally find Trdufelspitzen on the smaller herbs, as Melampyvum 

 sylvaticum which has long tapering pointed leaves, they are 

 very frequent on plants of a larger growth, especially on trees 

 and shrubs that grow by preference in shady woods or along the 

 banks of rivers, such as the ash and the willow ; those that seek 

 a drier soil, different kinds of Primus, roses, etc., have no such pro- 

 vision. The same difference in structure appears in a more pronounced 

 form when we compare the vegetation of different climates. In the 

 Eastern United States, where great heat alternates with heavy 

 thunder showers, there is a great abundance of tropical forms, much 

 more so than in Central Europe where the summer is drier and colder; 

 but even within the limits of the same genus, the American species 

 have distinctly longer pointed leaves than their European relatives. 

 In Europe, Stahl says, he noticed that longer points were developed on 

 leaves in moist regions than where the ground and atmosphere were drier. 



In further support of these observations, Stahl made a careful 



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