6 



IXTEODUCTORY. 



across a stalk of Indian Corn ; these steins are called Endogenous or 

 Emlogens, (inside grow^^rs). The terms Dicotyledonous and Exogenous 

 are used synonymously, as are Monocotyledonous and Endogenous. 



20. The Leaf. It is in the leaf that the important work of trans- 

 forming the crude sap, which is taken up by tlie roots, into organized 

 material fit to enter into the growth of the plant, is performed. In this 

 jDrocess the agency of sunlight is required and a free exposure to the air, 

 hence the leaves are so made and so disposed upon the stem as to present 

 the greatest possible surface to these influences. A leaf, haying all its 

 parts, consists of an expanded portion, (the Blade, Lamina, or Limb.) a 

 stalk by which it is attached to the stem, (the Petiole or Leaf-stalk.) and 

 a pair of appendages at the base of the leaf-stalk, called Stipules. The 

 petiole and stipules may one or both be absent, the essential portion 

 being the blade. Leaves having a petiole are said to be petioled or pe- 

 tiolate ; without a petiole they are sessile. "Where the blade joins the 

 petiole, or, if this be absent, the stem, is its base ; the opposite ends are 

 the apex, and the sides are the margins. 



21. The blade of the leaf consists of a green pulpy substance through 

 which runs a framework of fibres to give it strength ; these, as they are 

 large or small, are called Ribs or Veins, and the mode in which they are 

 distributed is termed venation. There are two principal kinds of vena- 

 tion : 1st, where the veins run mostly parallel, and do not branch nor 

 form a network ; these are parallel-veined (sometimes called nerved) 

 leaves, and are mostly to be found in endogenous plants (19) ; 2d, where 

 the veins form a sort of network through the pulpy portion ; the leaf is 

 then said to be netted- or reticulately-veined. This kind of veining has 

 two forms : 1st, where a strong rib, (the midrib), runs from the base to 

 the apex of the leaf, from which lateral veins branch off like the plame 

 upon a feather ; this is called feather-veined, or penni-nerved (Fig. 66) ; 

 2d, where several strong ribs start from the base and spread like rays 

 from the centre ; here we have a radiately-veined leaf (Fig. 68) ; these, 

 from their resemblance to a web-foot, are also called paJmately-veined. 



22. Feather- veined leaves are usually longer than broad, while in the 

 radiately-veined the form approaches the circular. The general outline 

 of leaves, as well as that of other flat portions of plants, is described by 

 a variety of terms. A very narrow leaf with two parallel margins is 

 linear, as the leaves of most grasses (Fig. 260) ; when the blade tapers 

 upwards or to each end, and is several times longer than broad, it is lan- 

 ceolate (Fig. 179) : when broader in proportion, o6/o)2g ; if both ends are 

 rounded and of equal width, elliptical ; when having the form of a hen's 

 egg cut lengthwise, with the broad end down, it is ovate ; when nearly 

 round, orbicular (Fig. 44). If the leaf tapers towards the base instead 

 of towards the apex, it is oblanceolate and obovate, the reverse of lanceo- 

 late and ovate. If rounded above and long and narrow below, it is 

 spatulate, and cuneate when shaped like a wedge. 



23. "When the two sides of the base are prolonged and rounded, the 

 leaf is said to be cordate or heart-shaped (Fig. 171) ; if such a leaf be 

 much broader than long, it is hidney-shnped or reniform. If the pro- 



