486 



THE POPULAR SCIENCE MONTHLY. 



" Flowers," has pointed to the power which, as Tyndall has shown, the 

 spray of perfume possesses to bar out the passage of heat-rays, and has 

 suggested that the emission of essential oils from the leaves of many 

 plants which live in hot climates may serve to protect themselves 

 against the intensely dry heat of the desert sun. 



Fig. 27. 



Fig. 28. 



I am rather disposed to think that the aromatic character of the 

 leaves protects them by rendering it less easy for animals to eat them. 

 In still drier regions, such as the Cape of Good Hope, an unusually 

 large proportion of species are bulbous. These, moreover, do not be- 

 long to any single group, but are scattered among a large number of 



very different families : the bulbous 

 condition can not, therefore, be ex- 

 plained by inheritance, but must 

 have reference to the surrounding 

 circumstances. Moreover, in a large 

 number of species the leaves tend to 

 become succulent and fleshy. Now, 

 in organisms of any given form the 

 surface increases as the square, the 

 mass as the cube, of the dimensions. 

 Hence, a spherical form, which is 

 so common in small animals and 

 plants, and which in them offers a 

 sufficient area of surface in propor- 

 tion to the mass, becomes quite unsuitable in larger creatures, and 

 we find that both animals and plants have orifices leading from 

 the outside to the interior, and thus giving an additional amount of 

 surface. But in plants which inhabit very dry countries it is necessary 

 that they should be able to absorb moisture when opportunity offers, 

 and store it up for future use. Hence, under such circumstances fleshy 



Fig. ^9. 



