194 The Leaf— Its Functions, etc. [May, 



sides of the leaf. On the under face we find the stomata or pores in 

 largest quantity, sometimes exclusively. This surface is generally 

 rougher and more downy than the upper. It is the reverse with leaves 

 floating on the water. The stomatia are on the superior surface. — 

 There are in these no fibro vascular system but merely an aggregation 

 of cells resembling veins, indeed there is no true epidermis. 



The parenchyma of the leaf exhibits two series of cells different 

 in form and arrangement. An ordinary leaf in its fullest develop- 

 ment consists of an expanded portion called the blade, and a nar- 

 rower portion called the stalk or petiole which is continuous with the 

 midrib, and sometimes of a portion of the petiole forming a sheath, 

 the latter again may be developed in the form of small leaves called 

 $tipula. 



Leaves are simple or compound. The simple leaf has but one 

 articulation with the stem, and the incisions in the margin do not 

 reach the midrib. 



The compound leaf has other articulations, beyond that with the 

 stem, or consists of leaflets separately attached with the petiole. 



If the petiole branches out into several parts, and each part con- 

 tains a set of compound leaves, they are called decomjiound, or 

 doubly compound. The simply compound leaves have two principal 

 modifications, depending on the position of the leaflets that com- 

 pose them. Thus, sometimes all the leaflets proceed from the very 

 top of the common petiole, as in the horse-chestnut ; sometimes 

 again they spring from the sides of the common petiole as in the 

 ash. 



The lower surface of leaves has numerous progecting prolonga- 

 tions running in different directions, which are divisions of the 

 petiole named nerves ; one of these nerves the midrib, or middle 

 nerve, is nearly constant in its disposition, and forms the continua- 

 tion of the petiole, having generally a longitudinal direction, and 

 dividing the leaf into two lateral parts which are pretty generally 

 equal. From its base and side proceed other nerves running in 

 difierent directions and frequently uniting with each other. The 

 nerves assume different names according to their thickness, and the 

 degree in which they project on the lower surface of the leaf. The 

 nerves, properly so called, are prominent and very distinct. AVhen 

 less prominent they are named veins ; and the last ramifications of 

 the veins, which intermingle frequently, and form the skeleton of 

 the leaf, are called veimles, or veinlets. 



