28 THE STORY OF PLANT LIFE 



or ring around a stem or branch, usually at a node. 

 The leaves may be opposite and when successive 

 opposite leaves are at right angles they are decussate. 

 They may be alternate and spiral. The amount of 

 divergence in the leaves or phyllotaxy can be 

 measured in terms of the circumference as a fraction, 

 being h in the Elm, where a spiral between two 

 successive leaves winds round half the stem. When 

 of two leaves this is indicated in the denominator 

 of the fraction as, for example, ^ or f, as in the 

 Peach. The numerator expresses the number of 

 turns. Numerous other types occur. 



Leaves thus exhibit almost as much, if not more, 

 diversity in form, arrangement, etc., as the flowers or 

 floral leaves, but for other purposes. Some leaves 

 are peculiarly modified to serve as pitchers, as in the 

 Teasel, some serve as spring traps as in Dionsea, and 

 for catching prey as in the Sundew. In other cases 

 they exhibit sleep movements as in Woodsorrel, or 

 respond to touch as in the Sensitive plant. 



The leaves vary in size according to the strength 

 of the stem, being large in trees, small in herba- 

 ceous plants.* They may be close together and 

 overlap as in the Beech, or be distant as in Hazel. 

 These facts are related to the procuring of the 

 greatest amount of light and air for photosynthesis.t 



* Though the reverse may be the case. Number, moreover, 

 counts in the case of trees. 



t The form and arrangement of leaves, in a mosaic, etc, and 

 therefore the form of each, as well as the distribution of the veins, 

 the form of outline, the structure of the petiole, and the existence of 



