THE LEAF. 



formed of three strata of greatly thickened cells so that 

 exhalation is completely checked, and the loss of moisture 

 effectually restrained. Nor is this all The epidermis, 

 you will remember, is pierced at regular intervals by 

 openings called 



255. STOMATA. These are the ORGANS OF EXHALA- 

 TION. They are situated so as to open directly into 

 the air-chambers of the intercellular system by which 

 they are surrounded ; and through them a circulation is 

 kept up between the cells of the interior of the leaf and 

 the outer air. In fig. 20, two oval bodies represent the 

 stomata. Each one, as will be seen, is composed of two 

 lip-like processes. These are constructed on the principle 

 of a self-acting valve, and are exceedingly sensitive to 

 the influence of moisture. When there is an excess of 

 water in the system, they elongate themselves, curving 

 outwardly, and thus open a passage for the exhalation of 

 the superfluous water. But when there is a deficiency 

 of water, and they become dry, they shorten and straighten 

 themselves, so as effectually to close the passages, and 

 thus prevent exhalation. The very circumstance which 

 is caused by a W:ant of water in the system, closes the 

 door against its useless expenditure, at the very moment 

 when the waste would become injurious. This sensibility 

 in the vegetable system, doubtless, to a considerable 

 extent, supplies the place of sensation, which belongs to 

 animals. 



256. POSITION OF THE STOMATA. These organs 

 are generally too delicate to bear the full power of the 

 sun, and hence they are chiefly found in the lower sur- 

 face of the leaf, where they are sometimes so numerous, 

 that 120,000 have been counted in a square inch of sur- 

 face. In sword-shaped, and other vertical leaves, in 

 which the two sides are both presented to the light, the 

 stomata are equally distributed over their surfaces. In 

 leaves which float on the water, as those of the White 

 Pond Lily, the stomata are all on the upper surface ; 

 and in completely submerged leaves they are not found. 

 There are no stomata in the epidermis of Roots. 



PRE FOLIATION. 



257. The peculiar manner in which the leaf lies 

 folded in the bud is called PREFOLIATION. The differ- 

 ent ways of folding in the leaves of plants, are so uni- 

 form, as to be of some importance in the determination 

 of their relationships. This subject at least furnishes a 



What in vegetables analogous to sensation in animals? Position of the Sto- 

 inata. Numbers. How in the upper surface ? Under ? How in sword-shaped 

 caves? In floating leaves? In submerged leaves ? How in the Root ? For 

 what are all these guards designed? Mode of folding in the bud? How 

 named? What buds recommended for study? What two points to be con- 



very interesting study ; and it may be pursued, in the 

 happiest manner, when the leaf-bud is completely swelled, 

 just before its expansion. Make a cross section with a 

 sharp knife, and the mode of arrangement will be exhib- 

 ited in the clearest and most beautiful manner. The 

 buds of the Apple, Peach, Horse-Chestnut, Lilac and 

 Walnut, may be recommended to the young student as 

 good subjects of observation. In studying the arrange- 

 ment of leaves, two things are to be observed ; first, the 

 manner of folding in the single leaf; and secondly, 

 the arrangement of the leaves in regard to each other. 



258. The following are some of the principal forms of 

 Prefoliation in the single leaf : 



(1) CONDUPLICATE, when the leaf is folded toward 

 the mid-vein, with the upper surfaces inclining toward 

 each other, as in the Oak, Peach and Magnolia, fig. 10 ; 



(2) CONVOLUTE, when the leaf is rolled up from one 

 of its edges in a complete coil, as in the Plantain, Cherry 

 and Apricot, fig. 13 ; 



(3) REVOLUTE, when both edges are rolled outwardly 

 toward the mid-vein, as in the Willow and Azalea, fig. 1 1 ; 



(4) INVOLUTE, when both edges are rolled inwardly 

 toward the mid-vein, as in the Apple, Violet and Water- 

 Lily, fig. 12. In the beautiful Lotus, a Water-Lily of 

 Egypt, fig. 21, the manner of folding in the involute leaf 

 is clearly shown ; 



(5) PLAITED, when the leaf is folded like a fan, as in 

 the Currant, Maple and Vine, fig. 14; 



(6) CIRCINATE, rolled up like a crosier, from the apex 

 to the base, as in the Linden and Fern, fig. 15 ; 



259. Secondly, of the folding of leaves in relation to 

 each other, the following are the principal varieties : 



(1) OBVOLUTE, when a half of each leaf embraces a 

 corresponding half of the next, as in the Sage, fig. 16; 



(2) SUPERVOLUTE, when one convolute leaf, embra- 

 cing another, is rolled up along with it, as in fig. 17 ; 



(3) EQUITANT, when the leaves successively inclose 

 each other in parallel lines, without any inversion, as in 

 the Iris, fig. 19; 



(4) IMBRICATED, when the outer leaves are succes- 

 sively folded over the inner, as in fig. 1 8. 



In the order of their folding in the bud, leaves ex- 

 hibit their future arrangement on the branches. 



260. ARRANGEMENT OF LEAVES. In respect to their 

 distribution on the stem and branches, leaves are 



(1) ALTERNATE, when there is only one to each 



sidered ? Forms of Prefoliation. Define and describe each. Are the six first 

 modes of folding in the single leaf, or in associated leaves? Define and do- 

 scribe the four next. How are these, single or associated fornrs ? Instances of 

 each mode. What modes of distribution on tho branches? How with one 

 leaf to each node ? 



