56 



PART I. THE MORPHOLOGY OF PLANTS. 



[12 



. Portion of 

 a leaf of Salix caprea 

 with reticulate vena- 

 tion : m mid-rib ; n the 

 larger lateral ribs; v 

 the anastomosing veins 

 ,(uat. size). 



veins branch repeatedly at various angles, and the branches for the most part 

 anastomose (Fig. 35). Some of them, however, end blindly in the meshes of 

 the network. This kind of venation is characteristic 

 of Dicotyledons; but it also occurs in some Mono- 

 cotyledons (e.g., Paris, Dioscorea, Smilax.many Araceae) 

 and many Ferns. 



The Different Forms of Leaves. The leaves 

 of different plants, as might be expected, are 

 not alike, but differ more or less widely in 

 size, shape, colour, and textnre. Bat even the 

 leaves borne on one and the same plant are 

 not all alike, the reason of their dissimilarity 

 being that, as there are different functions to 

 be performed, the leaves are variously adapted 

 in form and structure to the performance of 

 special functions. Further than this, the 

 same leaf may present different successive 

 forms, or a difference of form in its various 

 parts, because it may perform different func- 

 tions, either at different times, or simul- 

 taneously by its different parts. 

 1. Foliage-leaves are usually known simply as leaves (Fig. 36 L). 

 They are conspicuous on account of their green colour, and in 

 accordance with their nutritive function (see Part IV.), they are 

 expanded as much as possible to the sun-light. If they are small 

 they are very numerous (Conifers), and the larger they are the fewer 

 they are (Sun-flower, Paulownia). They nearly always possess a 

 well-developed lamina, which presents the various peculiarities of 

 conformation previously described. 



The texture of the leaf is dependent upon the mode and dura- 

 tion of its existence. The texture of most leaves may be described 

 as herbaceous. Leaves of this kind last usually for only a single 

 season, and die or fall off in the autumn. Leaves of firmer 

 texture, which are said to be coriaceous, survive the winter, and 

 cither, fall off when the new leaves are developed (the Privet), or 

 continue to live for several years (Holly, Box, and most Conifers) ; 

 the acicular leaves of the latter may persist for as many as twelve 

 years (Silver Fir). Fleshy or succulent leaves occur in plants 

 inhabiting dry regions or positions, such as Aloe, Sedum, etc. 



It is worthy of note that foliage-leaves of different form some- 

 times occur on the same shoot. For instance, it is commonly the 



