FOLIAGE-LEAF 69 



The parts of the leaf are of very varied form. In the grasses 

 the sheath completely embraces the stem, and in the Umbelliferse 

 (e.g. carrot, parsnip, and celery) it is very prominent ; in many 

 plants it is scarcely visible. 



The petiole where present is usually narrow and cylindrical ; 

 frequently it is very short or missing altogether, in which case 

 the leaf is described as sessile. 



The Made is generally the most obvious part of a foliage-leaf 

 and the points of importance to notice at present are its venation, 

 outline, margin, apex, and character of its surface. 



(a) Venation of leaf-blade. — The substance of the leaf is 

 traversed by a number of woody strands which are termed 

 veins or nerves, although it must not be inferred that they 

 are similar in structure or function to the veins or nerves of 

 animals. The arrangement of these strands is termed the 

 venation of the leaf, of which there are two common types, 

 namely (i) parallel and (2) reticulate or net-venation. In the 

 first type the chief strands all run parallel to each other from the 

 base of the leaf to the tip, as in the leaves of grasses, onion, 

 hyacinth, lily-of-the-valley, and Monocotyledons generally. 



In net-veined leaves the smaller very delicate strands form a 

 fine net-work within the leaf and this arrangement is charac- 

 teristic of Dicotyledons. 



Of reticulate veined leaves two divisions are made according to 

 the arrangement of the main strands.- In one, the leaves have a 

 central strand or mid-rib running down the middle of the leaf and 

 from it are given off slightly smaller branch strands as in Fig. 

 29; such leaves are pinnately veined or feather-veined, those 

 of the apple, plum, and peach are good examples. 



In the other division each leaf has several strong strands which 

 start from the base of the blade and spread across to its margins 

 somewhat like the fingers of an outstretched hand ; such a leaf is 

 described aspalmately veined. Ivy, sycamore, and currant leaves 

 show this type of venation. 



