90 



LEAVES 



veined ones are longer than they are wide. These differences 

 are particularly noticeable in leaves in which the leaf blade is 

 not all of one piece, divided leaves (Figs. 95, 96). 



Usually veins, near their origin, follow a pretty straight 

 course. This is desirable, in order to carry water as speedily 

 as possible from the base of the leaf to its tip. The arrange- 

 ment of the veins in the leaves of most land 

 plants is admirably adapted to strengthen 

 the leaf and protect it from being torn. 

 In many cases the last-named result is 

 secured by a sort of "binding" of looped 

 veins running around the margin, as is 

 fairly well shown in Fig. 94. 



108. Description of leaf forms. The 

 various forms of leaves are classed and 

 described by botanists with great minute- 

 ness, 1 not simply for the study of leaves 

 themselves, but also because in classify- 

 ing and describing plants the characteristic 

 shapes of the leaves of many kinds of plants 

 form a simple and ready means of distin- 

 . 94. Netted vein- guishing them from each other and identi- 

 ing (pinnate) in leaf fying them. 



of foxglove 109 occurrence of netted or parallel 



After Pianchon Dining. With few exceptions, the leaves 

 of monocotyledonous plants are parallel-veined and those of 

 dicotyledonous plants netted-veined. 



The needle-like leaves of the pines, spruces, firs, larches, and 

 other coniferous trees have but a single vein, or two or three 

 parallel ones ; but in their case the veining could hardly be other 

 than parallel, since the leaves are so narrow that no veins of 

 any considerable length could exist except in a position length- 

 wise of the leaf. 



1 See Kerner and Oliver, Natural History of Plants, Vol. I, pp. 623-637. 

 See also Appendix to this book. 



