580 



BOTANY. 



(petiole], though sometimes this member is wanting, 

 in which case they are said to be sitting (sessile). The 

 footstalk is either short or very long ; somewhat 

 grooved on the upper side, and convex below ; it is 

 produced into the web or disk of the leaf, where il 

 forms the mid-rib (costa), whence branches like veins 

 are sent off, traversing the whole disk. This dispo- 

 sition, and lateral branching of the mid-rib only, 

 obtains in dicotyledonous plants : the leaves of the 



Dicotyledonous leaf. 



class Monocotyledonces have the footstalk more 

 equally divided at the point where it enters the 

 disk, and thence projected towards the point of 

 the leaf in nearly parallel lines, fig. 14. 



The 



Monocotyledonous leaf. 



v/eb of the leaf is filled up between the veins by 

 cellular tissue, having a thin porous cuticle both 

 above and below. The structure of the upper and 

 under surfaces of the leaves are not alike ; one is 

 supposed to be furnished with excretory, the other 

 with incretory organs. Water plants, whose leaves 

 are constantly submersed, have no cuticle. 



Leaves are either simple, as in the myrtle, or com- 

 pound, as in the mountain ash. Compound leaves 

 are distinguished into pinnate, bipinnate, tripinnate, 

 ternate, biternate, triternate, and digitate. 



A pinnate leaf is said to be e'qual, when the leaflets 

 are opposite to each other on the footstalks ; alternate, 

 when placed one above another ; terminal, when the 

 primary footstalk has a single leaflet at its apex ; 

 cirrhose, when it is terminated by a tendril ; abrupt, 

 when it is terminated by a pair of leaflets ; inter- 

 rupted, when the alternate pairs are long and short ; 

 articulate, when the footstalk is jointed between the 

 pairs of leaflets ; and decurrcnt, when the leaflets have 

 u tnembranaceous continuation down the foot-stalks. 



A bipinnate leaf is when the footstalk itself is 

 divided, and branching out into other footstalks, 

 each of which supports leaflets corresponding to the 

 simply pinnate leaf in all its modifications. 



A tripinnate leaf is of the same description as the 

 above, once more decompounded. The petiole being 

 bipinnate, supporting 1 leaflets as in the former modes.i 



A ternate leaf is so called, when the footstalk 

 supports three leaflets, as is exemplified in the wood- 

 sorrel. 



A biternate leaf is when the common footstalk 

 supports three others, each of which supports three 

 leaflets. 



A triternate leaf is only a further decomposition ol 

 the above. 



A digitate leaf is when the common petiole sup- 

 ports several leaflets, diverging from a common 

 centre at its apex. 



Leaves are either radical, i. e. proceeding from the 

 crown or radical plate, or caulinar, 5. e. borne on the 

 stem ; and either sessile or petiolate, that is, either 

 sitting, or having footstalks. A sessile leaf is some- 

 times vaginant, that is, sheathing, as in grasses, or 

 amplexicaulc, stem-clasping, as in many of the umbel- 

 liforae ; or connate, situate opposite each other, and 

 united at the base, circumscribing the stem, as in the 

 leaves of the honeysuckle. In some cases connate 

 leaves form cups, which hold rain, probably for the 

 supply of the plant, or they are decurreiit, running 

 stem, as sow-thistle. 



The leaves are the principal organs of respiration, 

 and contribute to the growth by their powers of ab- 

 sorption. Gaseous qualities are emitted as well as 

 inhaled by them ; and that they allow the escape of 

 aqueous fluids is well known. As they aggregately 

 present an extensive surface to light and all atmos- 

 pheric influences, their action in the development of 

 the plant is indispensable. As such, they are with 

 great propriety called "the lungs" of vegetables. 

 They are mainly instrumental in producing a constant 

 flow of sap upwards, in consequence of their perspiring 

 functions. In proportion to the quantity of foliage, 

 in like proportion is the need of, and consumption of 

 water. Indeed it is not too much to assume, that 

 hydraulic action is their chief office : the amplification 

 of the system could not take place without such 

 agency, to excite intestine motion of the fluids, and 

 thereby assist the enlargement of the various mem- 

 branes. The health and vigour of every plant 

 depends very much on the number and amplitude ol 

 the leaves. Defoliation, either naturally, or by art or 

 accident, instantly arrests the growth, and the failure 

 or diminished expansion of foliage, is a certain sign 

 of debility. There is a numerous description of 

 plants called succulents, which have few or no leave?, 

 as the torch and melon thistles ; but their stoins are 

 much dilated, presenting a large superficies of paron- 

 chymous exterior to the air and light ; or they are 

 profusely covered with spines, which no doubt, con- 

 jointly, do the office of leaves. 



Green is the most general colour of leaves, but 

 some are red, or purple, or yellow ; some appear 

 nearly white, in consequence of being clothed with 

 short woolly or silky hair. They differ much in sub- 

 stance and structure ; some are immensely thick and 

 fleshy, as those of the genus aloe, others remarkably 

 thin, as those of the beech. The texture of the sur- 

 face is also very dissimilar ; some are rough, prickly, 

 and wrinkled, others smooth and glossy. In some 

 leaves, the middle rib and its ramifications or veins 

 are flat, on others remarkably prominent below. 



The figure of leaves are extremely various, and 

 sometimes constitutionally variable on the same plant, 

 as in the genns acacia; in which the bipinnate leavfs 

 of some of the species are changed into a single 

 oblong leaflet called Phyllodium, which is also altered 



