2o6 



LEAVES 



pinnate type. (The word means feather-like.) Most com- 

 mon trees and shrubs have pinnately-veined leaves. In 

 this type there is a central midrib from which branches are 



given off somewhat as harhs are 

 given off from the shaft of a 

 feather. (See Figure 6g.) 



Another common type of the 

 net-veined leaf is the palmate 

 type. (The word means palm- 

 like.) Palmate leaves have a 

 number of principal nerves or 

 veins which arise from the base 

 of the blade somewhat as spread 

 fingers arise from the palm of 

 the hand. The common plan- 

 tain has palmate leaves ; so has 

 the geranium. (See Figure yo) 

 A palm-leaf fan is an example of a palmate leaf of a 

 monocotyledon. 



Fig. 6g. — Twig of elm showing pin- 

 nate venation of the alternate 

 leaves and their toothed margins. 



B. Shape. — The shape of a leaf is a characteristic 

 which is not very constant. That is, the shapes of leaves 

 vary more or less as their surroundings vary. Thus, for 

 example, some plants which grow part in and part out of 

 water, such as the water hemlock, have submerged leaves 

 which are very different in shape and structure from those 

 which develop in the air. 



This tendency to vary in shape is not a property equally 

 possessed by all leaves. Leaves, like all other organs, 

 vary in their power to vary. Many plants have leaves of 

 distinct and characteristic shape which vary but little 

 and furnish a sure means of identifying the plant. Many 



