42 FOBMS OF LEAVES. 



Arrowhead, with its arrow-shaped leaves (Plate XVIL,fig. 3), 

 and the Lily of the Valley (Plate XVL, fig. 8), with its 

 pointed ovate leaves. This distinctive character of the leaves 

 is still more useful in deciding the class of a plant than the 

 form of the seed, because the leaves are more generally present. 

 A third point of difference exists, which can show the class 

 of a plant even when the leaves are absent, and that charac- 

 teristic is in the stem ; but we will speak of that when stems 

 are the subject of discussion. One remark more about leaves : 

 In distinguishing species it is useful to observe ih.e' margin of 

 the leaves. The simple pointed cutting on the edge of the Lime 

 leaf (fig. A), is called serrate. Where the points are deeper and 

 each point cut again, as in the Bramble (fig. D.), it is called 

 doubly serrate (or sawed, like the teeth of a 

 saw). Where little bits seem bitten out of 

 the margin of a leaf, as in the Hawkweed 

 (fig. K), it is called toothed; and where the 

 cutting is simple but the edges blunt, as in 

 the Mint (fig. H B), it is called notched. 



Some leaves are haired. Hairs serve as a 

 warm coat for the foliage, and they fulfil the 

 further purpose of collecting the moisture 

 from the air, and thus become doubly necessary for rock plants. 



The leaves are the lungs of the plant, and when they are 

 stripped off prematurely the plant sickens and often dies. 

 To the human race leaves are very important the shade they 

 afford in summer, the salutary effect of them cooked as an 

 article of diet, the refreshing tea, all these are claims upon our 

 gratitude at the present day. In the old times, of which Yarro 

 speaks, when the leaves of Palms and Mallows had to serve 

 the purpose of the convenient letter-sheet of to-day, and when 

 the Sibyl wrote her oracles on dry leaves and then scattered 

 them to the winds, they had a value of a different nature. In 

 India a leaf still serves as a tablecloth, and the Jesuit mis- 

 sionary of Manilla dwelt in a hut formed of two Palm leaves. 



