INTRODUCTION. 



.... Blade. 



the leaf, and there comes in contact with the air which enters through 

 the myriads of stomata little mouths or breathing pores which cover 

 its surface, and how, so far as we can see, the function is there performed, 

 are studies full of greatest interest. But it would be foreign to the 

 object of this text to further follow them now, however enticing. 



19. Nor can we in this place touch upon the interesting subject of the 

 differentiation of leaves for special purposes, such as for pitchers, fly- 

 traps, a means of support or defense, depositories for food, etc. All such 

 are found, and one can scarcely walk far in the fields in summer without 

 finding examples. 



20. We must here turn our attention to the forms of leaves and such 

 of their points as are important in the identification of trees. 



21. The Parts of a Leaf, if we examine one such as we would call 

 typical, perfect and serving only its primary purpose that of foliage 

 are three, as follows : (1) the blade or lamina, which is the flat and 

 expanded portion, with its surfaces ordinarily presented upwards and 

 downwards; (2) the stalk or petiole, which supports the blade on its 

 summit, and (3) the stipules, which are two usually small and more or 

 less leaf-like appendages at the base of the petiole, one on each side. 



22. Very commonly the stipules are absent when 

 the leaf is" said to be exstiputate. They often fall 

 away early, while the leaf is expanding, or they 

 may persist as little blades at, but quite distinct 

 from, the base of the petiole, or adnate to it making 

 wing-like expansions. Occasionally their margins 

 are united so as to form a sheath surrounding the 

 base of the petiole as seen in the Sycamore. In 

 color and substance they may be like the leaf, but 

 are often of different color, or colorless, and thin 

 and membraneous. 



23. Sometimes the petiole is absent, the blade 

 springing immediately from the stem of the plant. Such leaves are 

 called sessile, i. e., seated (from L. sedeo to sit.) 



24. The term "leaf," as ordinarily used, is applied to the blade, which 

 is its most conspicuous and important part. 



25. On further examination of our leaf we will see that it is made up 

 of a framework, consisting of a fibrous or woody material, and a softer 

 part, a green pulp, filling in the interstices. The framework is plainly 

 arranged, so as to give the leaf stiffness and support, and we speak of it 

 as consisting of ribs and veins. 



26. When there is a central main branch, it is known as the midrib, 

 and the branches leading off from this on each side are spoken of as the 

 veins, and their branches as veinlets. This leads us to the study of 



27. Venation, the arrangement of the ribs and veins of a leaf. 

 This is a subject of great importance, and, with other features which are 

 quite constantly associated, it enters as an important matter in the classi- 

 fication of plants. We shall see that there are primarily two grand sys- 

 tems of venation. The leaves representing one system are spoken of as 



28. Netted-veined or Reticulated when there are one or several main 



Fig. 4. A Simple, Pinnately-veined Leaf, showing parts. 



Petiole. 



Stipules. 



FIG. 4. 



