74 



PLANT BIOLOGY 



The method of compounding or branching follows the 

 mode of veining. The veining, or venation, is of two gen- 

 eral kinds : in some plants the main veins 

 diverge, and there is a conspicuous net- 

 work of smaller veins ; such leaves are 

 netted-veined. They are characteristic of 

 the dicotyledons. In other plants the 

 main veins are parallel, or nearly so, and 

 there is no conspicuous network ; these 

 are parallel-veined leaves (Figs. 89, 102). 

 These leaves are the rule in monocoty- 

 ledonous plants. The venation of netted- 

 veined leaves is pinnate or feather-like 

 when the veins arise from the side of a 

 continuous midrib (Fig. 91); palmate or 

 digitate (hand-like) when the veins arise 

 from the apex of the petiole (Figs. 88, 92). If leaves were 

 divided between the main veins, the former would be 

 pinnately and the latter digitately compound. 



It is customary to speak of a leaf as compound only 

 when the parts or branches are completely separate blades, 



Fig. 91. — Com- 

 plete Leaves of 

 Willow. 



Fig. 92. — Digitate-veined Pel- 

 tate Leaf of Nasturtium. 



Fig. 93. 



Pinnately Compound 

 Leaf of Ash. 



as when the division extends to the midrib (Figs. 90, 93, 

 94, 95). The parts or branches are known as leaflets. 



