16 BOTANY. 



4. Leaves and the Appendages. 



A leaf is a S3-mmetrical lateral expansion of the bark, and is intimately 

 connected with the internal part of the internal axis. Leaves, at first, are 

 mere projections of cellular tissue, closely united to each other ; subsequently 

 they enlarge -with the addition of vascular tissue, and finally assume a 

 permanent form and position along the axis. Whenever a leaf-bud is 

 formed, a leaf is also ; this, if not entirely developed, is at least rudimentary. 

 Two essential modifications of the leaf have been observed according as the 

 medium of existence is air or water. 



In aerial leaves we have a skeletal vascular tissue in the form of veins, 

 ribs, or nerves, the interspaces filled up by cellular tissue in the .shape of 

 parenchyma, the whole invested by epidermis. The vascular system is 

 continuous with that of the stem ; the vessels from the interior of the stem 

 spread out on the surface, the more external in the former appearing on the 

 inferior face of the latter. This is well illustrated by the fact, that in the 

 upper part of the leaf we find spiral vessels and woody fibre, in the lower 

 there are laticiferous vessels and fibres like those of liber. The vascular 

 system is distributed in the form of simple or branching veins. 



The epidermis generally diifers on the two sides of the leaf Thus, it is 

 on the under face that the stomata are found in largest quantity, sometimes 

 exclusively ; hairs also are of much more frequent occurrence. In leaves 

 floating on the surface of Avater the stomata are superior. The parenchyma 

 of the leaf is that cellular tissue filling up the interspaces of the vascular 

 fibre. Other names are diachyma and tnesophyllum. This parenchyma 

 exhibits two scries of cells, different in form and arrangement. 



Submerged leaves^ or those developed under water, exhibit many points 

 of difference from aerial leaves. There is here no fibro-vascular system, 

 but merely an aggregation of cells, which sometimes simulate veins. There 

 is no true epidermis, nor are there any stomata. Sometimes there is only 

 a net-work of filamentoid cells, the interspaces not filled with parenchyma. 

 Such leaves are called fenestrate. 



The ordinary leaf in its fullest form consists of an expanded flat portion, 

 called the blade, or laminar merithal ; of a narrower portion, called stalk, 

 petiole, or petiolary merithal, which is continuous with the midrib ; and 

 sometimes of a portion at the base of the petiole, forming a sheath or vagina ; 

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

 stipides: When a leaf has a distinct stalk, it is said to be petiolate ; when 

 there is no stalk, it is called sessile. When sessile leaves embrace the stem 

 they are said to be amplexicaul. The portion of the leaves next to the stem 

 is the base, the opposite extremity is the apex. The surfaces of the leaves 

 are called the paginoi ; the edges or margin, the circumscription. The usual 

 position of the leaf is horizontal ; sometimes it is vertical, or else between the 

 two positions. The upper angle, formed by the petiole with the stem, is the 

 axiUa ; anything arising from that part is axillary. The petiole is some- 

 times articulated with the stem, leaving a scar on the latter when it falls ; 

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