34 THE STORY OF PLANT LIFE 



in Dicotyledons and Monocotyledons, the veins in 

 the former being usually netted, in the latter parallel, 

 with some exceptions. But in the latter there are inter- 

 mediate veins connecting the parallel main branches. 



The structure of the vascular bundles of the leaf 

 is similar to that of the root and stem, but the 

 arrangements of the other tissues are different to 

 serve the purposes of photosynthesis, transpiration, 

 and respiration. 



The vascular bundles in the leaf are closed. The 

 wood lies nearer the upper surface, the bast nearer 

 the lower surface. The smaller strands have only 

 spiral tracheids, which continue to the tip of the leaf. 



The form of the stomata and their distribution 

 varies for each type of plant, and in special types of 

 plants, as aquatic plants, they are modified in rela- 

 tion to the environment. The general structure of 

 the leaf has already been described. In more cases 

 than in those of the root and the stem, and more 

 markedly, the leaf is adapted structurally to the 

 demands of the habitat. 



Also it must not be forgotten that the floral leaves 

 are specialised foliage leaves, serving a special pur- 

 pose. The structure of the flower affords differences 

 of form which are used as a basis for classification. 

 The internal structure of stamens and carpels is 

 more or less entirely connected with reproduction. 

 The various adaptations of flowers to insects, wind, 

 etc., have doubtless brought about not only their 

 difference in form and colour (in the case of the 



