THE LEAF. 



throughout the whole stem, the remainder of the Cam- 

 bium at length reaches the Root, which, it will he re- 

 membered, having no digestive organs, can elaborate no 

 nutriment for itself. To this it not only supplies new 

 materials of growth, but also, by enriching its juices, 

 enables it to maintain the action of Bndosmose, by which 

 new currents are continually put in motion. Thus the 

 circumnavigation of the Vegetable System is complete. 

 By a continued repetition of these processes, the tree 

 lives and grows year after year, until, in contemplating 

 its majestic proportions, we forget that the life of the 

 simplest herb that dwells beneath its shadow, is an illus- 

 tration of the same beautiful laws. 



CHAPTER XVIII. 



THE LEAP. 



249. Okganic Parts. — A leaf consists of two dis- 

 tinct parts — the flattened expansion called the Blade, 

 and the frame-work that supports it, which is the stalk 

 and veins. The first is composed of cellular tissue, the 

 latter chiefly of woody fibre. The leaf-stalk is called a 

 Petiole. When the petiole is not present, the leaf is 

 said to be sessile. 



anatomy of the leap. 



250 Leaves almost always expand horizontally, pre- 

 senting one surface to the earth, and the other to the 

 sky. This is their regular position ; and corresponding 

 with this, we commonly find a very marked difference, 

 both in color and texture, between the upper and under 

 surfaces, which is appreciable by the naked eye. By aid 

 of the microscope we find that the apparent diflferenoe 

 originates in the intimate structure of the parts. 



251. The Upper Sukpace is usually composed of a 

 single layer of oblong cells, very compactly arranged, 

 with their ends presented to the expansion, so as to leave 

 exposed the least possible extent of single walls and 

 intercellular spaces. But in plants which inhabit dry 

 and sterile regions, these superficial cells often consist 

 of two, and sometimes of three layers. The Melon, 

 which grows well in dry, sandy soils, has three of these 

 compact layers, as you will see at fig. 22 ; and the Ole- 

 ander, which is a native of Syria, has three layers also, 

 and still more compactly arranged ; while in the structure 



What effect? What chiefly in the Bark? Where at last arrire? What 

 service to the Eoot ? Why does not the Eoot elaborate its own food ? 



General subject Of what parts does the leaf consist? The elementary con- 

 Btitnenfs of each. Name of the Leaf-stalk. When absent how is the leaf ? In 

 what direction do leaves usually expand? What surface to the light? How 



of the Epidermis (77), there are equally important guards, 

 which are even more under the influence of atmospheric 

 changes. This arrangement, and some other peculiarities 

 of structure in the leaf, are designed to check evapora- 

 tion, or regulate the expenditure of fluids by the consti- 

 tution and necessities of the plant. 



252. Lower Surface.— In this the cells are oval or 

 ovate, and very loosely arranged, so as to admit of many 

 intercellular spaces, as in fig. 22. But in leaves which 

 present their surfaces equally to the light, there is no 

 difference between them, as in the Iris and Pine tribes. 



253. Light acts healthily on the upper surface of 

 leaves, hurtfully on the under surface; and if by any 

 accident, as high winds, they become displaced, they / 

 appear to make a voluntary effort to regain their true 

 position. In this way, plants that stand continually 

 with oae side to a window, grow awry, being drawn 

 around, by the impulse of the leaves, in their determina- 

 tion to present their upper surfaces to the light. When 

 cultivators wish plants to be symmetrical, they turn them 

 frequently. Some very interesting experiments on this 

 point were made during the vegetation of the young 

 Bean plants, which served as models to illustrate Ger- 

 mination in our fifth plate. A vigorous young plant, 

 about four inches high, while standing near a light win- 

 dow (but not in strong sunshine), was observed to have 

 made a curve in the stem, in order to present the upper 

 surface of its leaves more completely to the light. It 

 was turned round, when in the course of an hour it had 

 straightened itself, and in another hour had made an 

 opposite curve. By this time candles were brought in, 

 when one of them being before the plant, in the course 

 of the evening it turned completely back, and made 

 another curve toward the candle, proving itself equally 

 sensitive to artificial as to natural light. 



254. The Epidermis, or outer integument, usually 

 consists of a single layer of flattened cells. But this 

 organ also, when evaporation is to be restrained, affords 

 the same safeguards as the cells beneath, having in many 

 cases two strata of cells, and in some cases three. In 

 the Cacti — a tribe whose fleshy, succulent stems perform 

 the office of leaves, which are entirely wanting — the epi- 

 dermis is of a peculiar structure. As these plants in- 

 habit dry sands, bare rocks, and the hottest and most 

 sterile situations, they are admirably fitted to retain for 

 a long time whatever moisture they may acquire, either 

 from the atmosphere, rain, or dew. The epidermis is 



affected by light ? What instance ? Structure of the upper surface. How in 

 natives of very dry soils? Why? Under surface. Name of the outer integu- 

 ment What openings? What organs arc they? On what principle con 

 stracted ? Describe tlie operation. 



