POSITION OF LEAVES. STIPULES. 33 



leaf. Sometimes there are no stomata on the superior surface, 

 and the arrangement of the cells of the paren'chyma is not the 

 same as beneath. In the thickness of the leaf there are, ordi- 

 narily, cavities or intercellular lacunse which contain air, and 

 communicate externally through stomata (Jigs. 9 and 10) ; some- 

 times we also find in the paren'chyma, glands or reservoirs of the 

 proper juices. 



[The distribution of the vascular tissue through the limb of the leaf is 

 termed its venation or nervation, because the course of the vessels (of which 

 these nerves are made up) have been supposed to bear some resemblance 

 to the distribution of veins and nerves in certain parts of the animal struc- 

 ture. The bundles of vessels constituting the nerves, maintain nearly a 

 parallel course in their passage through the petiole, and are closely con- 

 densed together; but on arriving at the limb they separate, and, as we have 

 seen, are distributed in various ways. It will be observed they may all be 

 referred to one or the other of two classes, called the angulinerve and 

 curvinerve arrangement.] 



25. The position of the leaves on the stem and branches varies 

 in different plants, and furnishes very useful characteristics to 

 botanists for the distinction of species ; sometimes they are oppo- 

 site, that is, they rise in pairs at the same point from two sides 

 of the stem or peduncle (Jig. 70) ; sometimes they are verticil- 

 late, that is, grouped, three or more together, around the same 

 part of the stem ; and at other times they are alternate, that is, 

 they arise separately at different points. 



26. It is remarked, also, that opposite leaves are almost always 

 so arranged that the different pairs cross each other. When they 

 touch each other at the base, instead of arising from the opposite 

 sides of the stem, they are called gemini, or geminate leaves. 



27. On the stems of many plants, we observe on both 

 sides of each leaf, small organs named stipules, which seem 

 to be very analogous 



to leaves, but their ; ,^ ,1 



nature is not fully ^ \ x ^ '* 



ascertained (Jig. 16, 



s,s). They are only / ; 



found in the dicotyle'- 

 donous plants, and 

 they sometimes re- 

 semble little leaves, p 

 at others, scales. Fig-. 16. STIPULES. 



Explanation of Fig. 16 : s,, stipules arising at the axil of the leaf, that 

 is, where the petiole joins the stem ; Z, leaf ; p. petiole ; st. stem. 



25. When are leaves opposite ? When are leaves said to be verticillate ? 



26. When are leaves geminate ? 



27. What are stipules? To what description of plants are they con. 

 fined ? What is their use ? 



