538 cook: morphology and evolution of leaves 



cylinder. Each sheath in turn encloses the terminal bud of the 

 shoot, and later encircles the stem. 



Three elements of leaf structure — blade, petiole, and stipules — 

 are recognized generally in manuals and textbooks. These are 

 convenient for purposes of description, but for understanding 

 the structure and evolution of leaves it is better to begin with 

 the sheath or with the sheath and the blade, the two elements 

 that appear to have been differentiated in advance of the others. 

 Sheaths have been looked upon as expanded petioles or united 

 stipules, instead of being considered as a primitive element. It 

 has seemed reasonable to suppose that the petiole and the stip- 

 ules have been derived from the blade, which in most plants is 

 the largest and most important part of the leaf, but a general 

 interpretation in accord with evolutionary facts is needed in the 

 study of structural variations of plants. 



THE PRIMITIVE FUNCTION OF LEAVES 



In plants like Equisetum, Ephedra, and Casuarina the inter- 

 nodes perform the vegetative functions. Leaves are represented 

 only by sheaths or scales which are mere appendages of the inter- 

 nodes, apparently of little use except for protecting the buds. 

 Palms and many other plants afford examples of internodes and 

 sheaths that have chlorophyll and stomata in the epidermal tis- 

 sues and share the vegetative functions with the blades of the 

 leaves. Cacti and specialized desert plants of other families have 

 very small or rudimentary leaves, thus reducing transpiration. 



It is customary to think of bud scales and similar organs as 

 leaves that have been specialized by reduction, but it may be 

 more correct morphologically to think of foliage leaves as enlarged 

 bud scales that have assumed the vegetative functions formerly 

 discharged by the internodes. The function of the bud scales 

 is older than are the present forms of leaves, and may date back 

 to the stage when the sheaths were simple cylinders, before the 

 development of more specialized forms of leaf structure. It is 

 unnecessary to suppose that the early types of seed plants lacked 

 bud scales. 



