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A TEXTBOOK OF THEORETICAL BOTANY 



show that such a high degree of uniformity may be due, as we shall see 

 below, to their leaves being in reality phyllodes, or flattened petioles, with 

 a venation which is characteristic of petioles or leaf bases rather than of true 

 laminae. The same is probably true of reduced leaves, such as bracts and 

 bud scales, which usually show a simple and parallel type of venation even 

 in Dicotyledons. 



The variety of patterns displayed by net-veined leaves is due, at least 

 in the principal features of the pattern, to differences in the mode of growth 

 of the leaf blade. In some families the pattern is so constant that it forms a 

 useful character for recognition. In other cases there may be variation even 



A B 



Fig. 948. — Pinnate and palmate \enation. A, Pinnate ; Hydrangea. 



B, Palmate ; Cercis. 



in different leaves of the same plant, a notable instance being the contrast 

 between the floating leaves and the submerged leaves of many aquatics. 



It has been observed by Zalenski that age affects the leaf structure. Com- 

 paring mature leaves produced earlier and later in the development of a 

 plant, the latter have smaller cells and consequently the vein islets are smaller 

 and the stomata and hairs are closer together. This is one of the few examples 

 of senescent changes of structure in plants. 



Two main varieties of pattern may be mentioned, namely pinnate and 

 palmate venation (Fig. 948). The former has a single midrib from which 

 the chief branches are given off in pinnate order. The latter usually has 

 several main veins springing from the upper end of the petiole and spreading 

 out fanwise. While both types are frequently associated with leaves which 

 show the corresponding type of division of the lamina, they may also occur 



