ON LEAVES. 55 



gether, forming the petiole or foot-stalk : thence they ex- 

 pand in numerous ramifications, constituting the ribs of 

 the leaf. Let us now examine this leaf of a horse-ches- 

 nut: I cut it transversely at its base, and you may per- 

 ceive with the naked eye the larger vessels which con- 

 vey the sap into the leaf. At the other extremity of the 

 foot-stalk they are also visible. They are five in num- 

 ber, corresponding with the five leaflets of which the 

 horse-chesnut leaf is composed. 



The fibres of leaves spread out in various directions : 

 the principal one, dividing the leaf from the base to the 

 summit, is called the dorsal, or midrib ; others branch out 

 from this laterally ; and a third class consists of still smal- 

 ler ramifications issuing from these last : they all termi- 

 nate at the surface of the leaf by a pore called stoina, a 

 Greek word, signifying mouth. 



Caroline. These are, no doubt, the exhaling pores 

 which send off the superfluous moisture. 



Mrs. B. Yes ; but we must patiently labor through a 

 forest of foliage, before we can return to the physiologi- 

 cal operations of the plant. 



Leaves are usually divided by botanists into five class- 

 es, according to the direction of their ribs : 



First, the pennated are those in which the smaller ribs 

 expand from the principal rib like the feathers of a quill : 

 the leaves of the pear and the lime-tree are of this descrip- 

 tion. 



The second class is palmated. In these, the ribs di- 

 verge from the petiole like the fingers from the palm of 

 the hand, as you see in this vine-leaf. They are not, 

 however, always five in number, varying not only in dif- 

 ferent plants, but sometimes in different leaves of the 

 same individual. 



The third class is called target-shaped, or peltate, be- 

 ing shaped like a buckler ; such is the nasturtium. 



The fourth class is pedatum, having the form of the 

 foot : the hellebore is of this class. 



280. What illustration is made with the leaf of a horse chesnut! 

 281. What is said of the vessels which convey the sap! 282. 

 What is called the dorsal? 283. What other fibres have leaves and 

 how do they all terminate! 284. How are leaves usually divided by 

 Botanists! 285. What is the first class! 286. What is the sec- 

 ond and how is it described! 287. What is the third class! 288. 

 And the fourth! 



