54 



ANALYTICAL CLASS-BOOK OF BOTANY. 



flowers, ■wholly void of beauty, in the Low Cornel, fig. 

 10, Plato XIX. In the Cornus Florida, a small tree of 

 the same family, and usually known by the name of Box- 

 wood, the four large white bracts are particularly beauti- 

 ful, and so delicately organized, as to appear, in a super- 

 ficial view at least, as the real blossom ; while in the 

 Calla, fig. 13, the large lily-white bract is the sole 

 adornment of that supei-b flower. 



297. In the Lime-tree, fig. 8, the bract, which bears 

 the fiowering stalk, is oval-lanceolate, and of a light yel- 

 lowish pea-grecu ; while the true leaf is ovate, or cordate, 

 and of a beautiful dark glossy color. The bracts here 

 also constitute the chief beauty of the tree, for they have 

 all tho effect of flowers, aud last during the whole season. 



298. The membranous scales of the Glume, which 

 inclose the spikelets in Grasses, are a peculiar form of the 

 bract ; and the scales of the flower itself are properly so 

 termed. A spikelet is seen at fig. 4, and one more ex- 

 panded at fig. 5, showing the palea3, or bracts that in- 

 close the single flower. 



299. In the great natural Order of Compositse, 

 which embraces the Compound Flowers of the older 

 Botanists, the heads are inclosed by leaflets, generally 

 numerous and narrow, which are also called bracts, as in 

 the Marigold, fig. 1. The scales of the ament, a, in the 

 Walnut, fig. 2, and in the Willow, fig. 3, are bracts, and 

 so are those of all amentaceous trees — a circumstance 

 that distinguishes them into a finely marked natural Order. 



300. Other Forms. — The cluster of leaves at the 

 summit of the fruit in the Pine-apple, fig. 6 ; those which 

 inclose the umbels and umbellets, in the UmbelliferEe, fig. 

 7 ; the thin scales of the Hop, fig. 11; as well as the 

 bony and indurated ones of the Pine-cone, fig. 9, and the 

 Oak-cupule, fig. 12, are all difierent forms of the bract. 

 There is no absolute distinction between this organ and 

 the proper leaf; and in their gradual transitions, they 

 sometimes offer good illustrations of the metamorpJioses 

 that occur in plants, of which you will hear something at 

 another time. 



CHAPTER XXI. 



FUNCTIONS OF THE LEAF. 



301. The fijst and most obvious use of leaves is that 

 of furnishing chilling to the plant, and thus protecting 



The Low Cornel. Calla. Lirae-trco. What part are they in Grasses, in the 

 Compos] t;o — Amentaceous trees — TJmbellifera; — Oak — Pine-Apple — Hop. 



General subject J^Iost obvious Function of the Leaf. What others? To 

 what organ does Absorption particularly belong ? How shown to e.xist in the* 

 Leaf; "What ■ facts ? By which surface do leaves chiefly absorb? How 



its more delicate organs from heat, cold, and other exter 

 nal injuries. But in addition to this, the oflices of the 

 Leaf are the most important and remarkable in nature. 

 The principal of these are. Absorption, Digestion, and 

 Exhalation. 



302. Absorption. — This is the distinctive function 

 of the Eoot ; yet in plants which have no root, this office 

 is performed by the leaves, as in Epiphytes (159), or 

 often by the whole plant, as in the Mosses and the lowest 

 orders generally. That this power is also, to a greater 

 or less extent, a property of all leaves, may be shown by 

 several facts. In the first place, plants will languish in 

 a very dry atmosphere, even when their roots are copi- 

 ously supplied with water. Second, the leaves of trees 

 and other plants, in times of drought, will revive sud- 

 denly after a shower, and that long before they could 

 receive water by transmission from the roots. Third, 

 when cuttings of plants are kept in vases, they will 

 retain their freshness much longer, if their whole surface 

 is sprinkled occasionally with a plentiful shower of 

 water. 



303. Leaves absorb chiefly by their under surface. 

 This is shown by the following simple experiment. 

 Place a number of detached leaves, of the same species, 

 in water, with different surfaces applied to the liquid ; 

 and those which are placed with the upper surface down 

 will wilt much sooner than those with the under sur- 

 face down. Wood, in his excellent " Classbook of Bo- 

 tany,'' says that leaves of the White Mulberry, placed 

 with their upper sm-face in contact with water, faded in 

 six days ; while those in the reversed position lasted as 

 many months. Leaves absorb gases from the air. 



304. Digestion. — This is a function precisely similar 

 to that of the animal economy. The crude sap having 

 reached the Leaf, traverses the green substance known 

 as Chlorophylle, which, as has before been hinted, in con- 

 nection with the solar light, produces some very remark- 

 able chemical changes. The crude liquid, of which car- 

 bonic acid gas and water are supposed to form the prin- 

 cipal portion, is decomposed ; the carbon is retained, as 

 the chief basis of the vegetable structure, while the 

 liberated oxygen, and the superfluous water, are returned 

 to the air. 



305. By this process the crude sap is reflned, and 

 is converted from its inorganic state to the material of 

 organism. This is one of the most wonderful results in 



proved— what experiment? What do loaves absorb from the air ? Vegetiible 

 Digestion, what does it resemble ? What auustance in tho leaf does sap tra- 

 verse ? What effect ? What substances chiefly form the crude liquid ? Wliat 

 becomes of each ? Into what is the crude sap converted? Wtiat wonderful 

 result of Vegetable Digestion ? 



