LEAVES 17 



rudimentary structures or may even be entirely wanting. The 

 leaf may also be without a petiole, in which case the blade is 

 directly connected with the stem or branch of the plant and is 

 then said to be sessile. 



25. The petiole is stiff enough to support the blade and yet 

 is flexible and elastic. It is composed largely of fibrous woody 

 tissue, which extends on from the petiole into the blade, where 

 it is so disposed as to constitute a framework upon which the 

 more delicate tissues of the blade are supported. This frame- 

 work is made up of one or a few large ribs which by branching 

 give rise to numerous smaller veins and veinlets. 



26. The veins may be arranged in one of two ways; they 

 either lie parallel with one another and extend from the base 

 of the blade to its tip or from the single large midrib to the 

 edge of the blade, or else they unite with each other in such a 

 way as to form a network. Leaves having the former arrange- 

 ment of the veins are said to be parallel veined, while those pre- 

 senting the latter condition are termed netted veined. Netted 

 veined leaves may further be characterized as feather veined 

 if there is only a single large midrib from which the principal 

 veins branch on either side, or palmately veined when there are 

 several ribs spreading in a fan shaped order from the base of 

 the blade. 



27. The upper and lower surfaces of the leaf blade are formed 

 by thin, transparent, but rather tough, membranes, the epider- 

 mis, which may be stripped off. Between the two layers of 

 epidermis lies the green mesophyll, which next the upper epider- 

 mis forms a rather firm tissue, but on the lower side is more 

 spongy in texture. 



28. In outline the leaf blade is extremely variable. All forms 

 from circular to narrow ribbon-like or even thread-like are met 

 with, and the margin varies from a continuous line or unbroken 

 curve to conditions which may be described as toothed, lobed, 

 cleft or divided, as the case may be. A divided leaf is one in 



