1890.] on the Shapes of Leaves and Cotyledons. 103 



stalk to blossom, they seem perpetually to tempt our watclifulness, 

 and take delight in outstripping our wonder." 



Now, why is this marvellous variety, this inexhaustible treasury 

 of beautiful forms ? Does it result from some innate tendency of each 

 species ? Is it intentionally designed to delight the eye of man ? 

 Or have the form and size and texture some reference to the structure 

 and organisation, the habits and requirements, of the whole plant? 



The leaf, although so thin, is no mere membrane, but is built up 

 of many layers of colls, and the interior communicates with the 

 external air by millions of little mouths, called stomata, which are 

 generally situated on the under side of the leaf. The structure of 

 leaves varies as much as their forms. 



It is, of course, principally in hot and dry countries that leaves 

 require protection from too much evaporation. 



The surface is in some cases protected by a covering of varnish, 

 in others by saline or calcareous excretions. In others, again, the 

 same object is attained by increased viscidity of the sap ; in some, the 

 leaves assume a vertical j)osition or range themselves one under the 

 other, thus presenting a smaller surface to the rays of the sun. In 

 other cases the leaves become fleshy. Woolly hairs are also a common 

 and effective mode of protection. The plants of deserts are very fre- 

 quently covered with a thick felt of hair. Some species, again, which 

 are smooth in the north, tend to become woolly in the south. Species 

 of the cool spring again tend to be glabrous. The uses of hairs to 

 plants are indeed very various. They serve, as just mentioned, to 

 check too rapid evaporation. They form a protection for the stomata 

 or breathing holes, and consequently, as these are mainly on the 

 under side of leaves, we find that when one side of the leaf is covered 

 with white felted hairs, as the white poplar, this is always the under side. 



In other cases the use of hair is to throw off water. In some 

 alpine and marsh plants this is important. If the breathing holes 

 became clogged with moisture — with fog, for instance, or dew — they 

 would be unable to fulfil their functions. The covering of hair, 

 however, throws off the moisture, and thus keeps them dry. Thus 

 these hairs form a protection both against too much drought, and too 

 much moisture. 



Another function of hairs, which cannot be omitted, is to serve as 

 shades against too brilliant light and too much heat. Again, hairs 

 serve as a protection against insects, and even against larger 

 animals. The stinging hairs of the common nettle are a familiar 

 example, and coarse woolly hairs arc often distasteful to herbivorous 

 quadrupeds. 



Deciduous leaves especially characterise the comparatively cool 

 and moist atmosphere of temperate regions. For different reasons 

 evergreen leaves become more numerous in the Alj)S and in the 

 tropics. 



In the Alps it is necessary for plants to make the most of the 

 short summer. Hence, perennial and evergreen species are more 



