OF LEAVES. 



39 



other hand, when it embraces the stem in its 

 whole circumference, as in the common goata- 

 beard, and the white poppy. 



Frequently also the base of the leaf is pro- 

 longed so as to form a sheath, which entirely 

 surrounds the stem, and envelopes it for a certain 

 length. In this case, the leaves are named 

 sheathing, as in the wheat, oats, and grasses. 

 This sheath may ba considered as a very broad 

 petiole, of which the two edges are occasionally 

 united to foi-m a kind of tube. The place at 

 which the limb of the leaf and the sheath meet, 

 is named the necJc. Sometimes it is naked, at 

 others furnished with hairs, as in poa pilosa, or 

 with a small membraneous upper appendage, as 

 is observed chiefly in the grasses. 



Leaves of the same plant frequently differ in 

 form, according to the part of the plant from 

 which they arise ; hence they are seminal leaves 

 when the cotyledons rise above the soil and as- 

 sume the form of leaves. These wiU be alluded 

 to when treating of the germination of seeds. 

 In some plants the diversity of the leaves is very 

 remarkable ; thus, ivy has some of its leaves 

 entire, and others deeply lobed. Leaves also 

 vary according to the medium in which they 

 vegetate. Aquatic plants have generally two 

 kinds of leaves ; one set swimming at the surface 

 of the water, or raised a little above its level ; 

 the other always immersed in this fluid. Thus, 

 the water crowfoot has lobed leaves which float at 

 the surface, and leaves divided into exceedingly 

 narrow and very numerous segments, which are 

 immersed in the water. Radical leaves are those 

 which come off directly from the root ; cauline, 

 or stem leaves, are attached to the stem, while 

 branch leaves spring from tlie minuter ramifica- 

 tions. Floral leaves are those which accompany 

 the blossoms. 



Leaves may be ranged opposite 

 to each other in pairs, or alter- ^'' 



nate, coming off one by one. 

 at nearly equal distances from^ '^ 

 different sides of the stem. Ver-'^'^ 

 ticillate or whorled, when more 

 than two come off together, thus 

 forming a circle around the stem, 

 as in the galiums. Sometimes 

 the leaf entirely surrounds the stalk, when it is 

 called perfoliate, d. 



The Frond consists of a union, 

 or incorporation of the leaf, leaf- 

 stalk, and branch or stem, forming, 

 as it were, but one organ of which 

 the constituent parts do not se- 

 parate spontaneously from one 

 another, by means of the fracture 

 of any natural joint, as in the 

 case of plants in general ; but ad- d. 



here together even in their decay. 

 The leaf of the palms was termed a frond by 



Linnseus ; and that of the ferns go under the 

 same name. The algce and lichens consist, for 

 the most part, of fi-onds only. 



Leaves are of various shapes, and have received 

 names accordingly; as, orbicular, oval, oblong, 

 lanceolate, linear, awl-shaped, filiform, cordate 

 or heai-t-shaped, reniform, or kidney-shaped, 

 sagittate, hastate. Leaves may also be deeply 

 notched, so as to be divided into three or more 

 lobes, hence they are called tripartite, quadri- 

 partite. Pinaiatified, divided into deep lobes 

 along the sides. The following figures wiU show 

 the most striking diversities of the form ol 

 leaves. 



Finnatified, Laciniate. 



Emarginate, 



Cordate. Tripohate. Lanceolate. Linear. Orbicular. 



Leaves are also called entire, 

 when the margin is even; den- 

 tate, when the margin is 

 marked by erect teeth. Ser- 

 rate, when the teeth are in- 

 clined to the summit of the 

 leaf, like the markings of a 

 saw. Doubly serrate, when 

 Trilobate. each smaU tooth is itself ser- 

 rated. Spinous with acute 

 thorns, or spines, as in the hoUy and thistle. 

 Ciliated, having the margin furnished with hairs. 

 Leaves are also distinguished by their external 

 surface into shining, smooth, coreaceous or 

 leathery, glutinous, fleshy, glaucous, or sea green, 

 spotted, &c. 



Compound Leaves. When several leaves are 

 attached to a common petiole, they are said to 

 be compound, fig. e. If the petiole branches out 

 into several parts, and each part contains a set 



