38 



HISTORY OF THE VEGETABLE KINGDOM. 



inwards or outwards, or the leaf is rolled fi-om 

 above downwards, as in the family of ferns. 

 Leaves are characterized as membranous, greenish 

 coloured, and generally flat appendages, springing 

 from twigs, branches, the trunk, or the neck of 

 the root of plants. They generally lie with 

 the plane of their surface in a horizontal direc- 

 tion. Their epidermis is porous, and their use 

 is to absorb gases and moisture for the nourish- 

 ment of the plant, and at the same time to exhale 

 the moisture and gases which are superabundant. 

 Leaves are formed of a net- work, of fibrous sub- 

 stance similar to that composing the stem, while 

 the interstices are filled up with cellular tissue, 

 or a soft greenish matter, similar to the herba- 

 ceous envelope which lies between the outer and 

 inner bark of the stem. 



Sometimes the body of the leaf is attached 

 immediately to the stem, without any inter- 



mediate stalk, as in the common poppy, when, 

 it is said to be sessile, fig. b. 



More frequently, however, there is a central 

 stalk on which it is supported, which is called 

 the petiole. In this case the leaf is said to be 

 petiolate, as in the elm, lime, rosemary, &c., fig. c. 

 This latter being the more general arrange- 

 ment, the leaf may be considered as formed of 

 two parts, the petiole, and the disk or limb ; the 

 latter being the generally flat and gi-eenish part 

 which constitutes the leaf properly so called. 

 Tlie petiole being wanting in many leaves, and the 

 limb itself being also sometimes absent, through 

 abortion; the leaf then consists of the petiole 

 only, which frequently dilates and assumes the 

 form and characters of a sessile leaf: as, for ex- 

 ample, in all the simple-leaved acacias of New 

 Holland. 



In the leaf there are distinguished an upper 

 surface, which is commonly smoother, more 

 green, covered with a more closely adhering 

 epidermis, and presenting fewer cortical pores ; 

 and a lower surface, of a less deep colour, often 

 covered with hair or down, with an epidermis 

 more loosely attached to the herbaceous layer, 

 and presenting a great number of small holes, 

 the orifices of the internal vessels of the plant. 

 It is accordingly by their lower surface that the 

 leaves absorb the fluids which are exhaled by 

 the earth, or which are diffused and mingled in 

 the atmosphere. 



The lower surface of the leaf is also remarka- 

 ble for numerous projecting prolongations run- 

 ning in different directions, which are merely 

 divisions of the petiole, named nerves. One 

 of these nerves, named the mid-rib, or middle- 

 nerve, is nearly constant in its disposition, 

 and forms the continuation of the petiole, having 

 generally a longitudinal direction, and dividing 

 the leaf into two lateral parts which are pretty 

 frequently equal. From its base and sides pro- 

 ceed the other nerves, running in different direc- 

 tions, and frequently uniting with each other. 

 The nerves assume different names according 

 to their thickness, and the degree in which they 

 project at the lower surface of the leaf. The 

 nerves, properly so called, are prominent and 

 very distinct ; when they are less so, they are 

 named veitis ; and the last ramifications of the 

 veins, which intermingle frequently, and fonn 

 the skeleton of the leaf, are called venules. 



It must be observed that the nerves of plants 

 have no resemblance in structure or functions to 

 the nerves of animals. They are merely bundles 

 of porous vessels, spiral vessels, and false trachioe, 

 enveloped in a cei-tain quantity of cellular tissue. 

 Sometimes the nerves are prolonged beyond the 

 circumference of the dish of the leaf, and when 

 rigid, form spines or thorns, more or less acute, 

 as in the holly. 



As the disposition of the nerves upon the 

 leaves serves to characterize certain divisions of 

 vegetables, they merit the greatest attention. 

 Thus, in most of the mowKott/ledones, the nerves 

 are almost always simple, very little branched, 

 and with few exceptions, parallel to each other, 

 as in cut 26, fig. a. In the dicotylcdones they 

 do not present this disposition; but they are 

 frequently much ramified, and interlaced with 

 each other. 



The more remarkable varieties in tlie disposi- 

 tion of the nerves may be referred to the fol- 

 lowing : 



A leaf, whether sessile or petiolate, may be 

 attached, in various ways, to the stem or branches 

 which support it. Sometimes it is simply arti- 

 culated, and without directly uniting by the whole 

 of its base, is simply fixed by a kind of contraction 

 orarticuIation,a3 in the maple and horee-chestnut. 

 These leaves are then caducous, or fall very early. 

 At other times the leaf is so united to the 

 stem, that it cannot be separated from it without 

 being torn. Such leaves remain on the tree as 

 long as the branch that supports them, as in the ivy. 

 The manner in which sessile leaves 

 are attached to the stem also deserves 

 examination. Thus the middle nerve 

 sometimes enlarges and embraces the 

 stem in about half of its circum- 

 ference. The leaves are then named 

 semiamplevicaul. 

 The leaf is said to be amplexicaul, on the 



