MORPHOLOGY OF THE LEAF. 91 



CHAPTER XX. 



MORPHOLOGY OF THE LEAF. 



271. The leaf constitutes the verdure of plants, and is by far 

 the most conspicuous and beautiful object in the scenery of 

 nature. It is also of the highest importance in the vegetable 

 economy, being the organ of digestion and respiration. It is 

 characterized by a 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 bark, extended into a broad thin plate by means of a woody 

 framework or skeleton, issuing from the inner part of the stem. 

 The expanded 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 apparent, 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. But they are 

 subject to endless transformations. 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 gen- 

 erally flattened or channelled on the upper side. When it is flat- 

 tened 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 icinged %)etiole 

 is flattened or expanded into a margin, but laterally instead of 

 vertically, as in the Asters. Sometimes the margins outrun the 

 petioles, and extend down the stem, making that winged, or 



