[Chap. X 



HEREDITARY DIFFERExNCES IN LEAVES 



87 



Fig. 45. Petiole and under side of leaf of Victoria regia. Photo from New York 



Botanical Garden. 



Structures. The illustrations in this chapter show some of the striking 

 differences in the structures of leaves. Simple one-celled hairs, or multi- 

 cellular branched, stellate, and shield-like hairs are characteristic of 

 certain leaves. The pine needle has peculiar chlorenchyma cells, and 

 resin ducts. In the aloe leaf the chlorenchvma is a peripheral layer en- 

 closing a mass of large, thin-walled gelatinous cells that have a high 

 water-retaining capacity. The leaves of many aquatics that extend or 

 develop above the water level are noted for the large proportion of air 

 cavities extending not only throughout the leaf but downward into the 

 stem and roots. The relative numbers of stomates and their occurrence 

 on the two sides of a leaf are characteristic of a given species ( Table 2 ) . 

 The number of stomates per square millimeter varies from place to place 

 on a particular leaf, and the average of several counts must be taken 

 when comparisons are made. 



