91,92.] MORPHOLOGY OF THE LEAF. 107 



thin and expanded form, presenting the largest possible 

 surface to the action of the air and light, which agents 

 are indispensable to the life and increase of the plant. 

 The leaf may be regarded as an expansion of the 

 substance of the stem, extended into a broad thin 

 plate by means of a woody frame-work or skeleton, 

 connected with the inner part of the axis. The ex- 

 panded portion is called the lamina or blade of the 

 leaf, and it is either sessile, that is, attached to the 

 stem by its base, or it is petiolate, attached to the 

 stem by a footstalk called the petiole. 



272. The regular petiole very often bears at its 

 base a pair of leaf-like appendages, more or less ap- 

 parent, called stipules. Leaves so appendaged are said 

 to be stipulate; otherwise they are exstipulate. 



273. Therefore a complete leaf consists of three 

 distinct parts the lamina or blade, the petiole, and 

 the stipules. These parts are subject to endless trans- 

 formations. Either of them may exist without the 

 others, or they may all be transformed into other 

 organs, as pitchers, spines, tendrils, and even into the 

 organs of the flower, as will hereafter appear. 



274. The Petiole in form is rarely cylindrical, but 

 more generally flattened or channeled on the upper 

 side. When it is flattened in a vertical direction, it is 

 said to be compressed, as in the Aspen or Poplar. In 

 this case, the blade is very unstable, and agitated by 

 the least breath of wind. The winged petiole is flat- 

 tened or expanded into a margin, but laterally instead 

 of vertically, as in the Orange. Sometimes the margins 

 outrun the petioles, and extend down the stem, mak- 

 ing that winged, or alate, also. Such leaves are said 

 to be decurrent (decurro, run down). Ex., Mullein. 



