186 LEAVES FROM THE BOOK OF NATURE. 



into tendrils, the pea into hands with three or five fingers, 

 with which to grasp its support, though only when it has 

 reached a certain height, and needs the latter ; the passion 

 flower converts them into cork-screws, whilst the common 

 nasturtium is content with a simple hook at the end of 

 the leaf. Their arrangement also around stem and branches 

 is not left to accident: a distinguished mathematician of 

 our Cambridge once astonished a large and learned au- 

 dience not a little, when he informed them that plants 

 knew mathematics, and arranged their leaves according to 

 fixed rules. A spiral line drawn from the base of one 

 leaf, around the stem, to that of another, shows regular 

 intervals between them, which vary in different plants, but 

 are in each carefully and strictly observed. 



The great purpose of life in leaves is to carry on their 

 most active and important vital function their respiration. 

 They are the lungs of plants, not condensed, as in man, 

 in one organ, but scattered independently in countless num- 

 bers over the branches. For the purpose of breathing 

 they are endowed with innumerable and often invisible 

 little openings, commonly on both sides in aquatic plants, 

 however, whose leaves float on the surface of the water, 

 only on the upper side. In the cactus tribe these are al- 

 most wholly wanting, hence the latter are so succulent, 

 as they retain all the fluid that their roots have sucked 

 up, and exhale nothing. Their activity is, of course, a 

 twofold one, as they both take in and give out, with- 

 out ceasing. They inhale atmospheric air, appropriate its 

 carbon for the formation of their juices, and return the 

 separated and disengaged oxygen in the form of gas. This 



