198 SAP AND SECRETIONS. which are seen to best advantage in the centre. These cells, which are unusually large in the Elder, are filled with fluids when young, but in old branches the fluids are gone and the cells are empty. Of its uses in the economy of vegetation, but little is known. QUESTIONS.-1. What is the cuticle of a plant? 2. How is it deseribed and what are its uses? 3. Describe the cellular integument. 4. The bark. 5. The wood. 6. The pith. 7. What chiefly resides in the bark of plants? 8. What is said of the circular layers of wood? 9. How has it been shown that the bark produces the wood? LESSON 89. Sap and Secretions. Odoriferous, fragrant, perfumed. Propulsion, the act of driving forward. Es'culent, good for food, eatable. THAT the whole vegetable body is an assemblage of tubes and vessels is evident to the most careless observer; and those who are conversant with the microscope and books relating to it, have frequent opportunities of observing how curiously these vessels are arranged, and how different species of plants, especially trees, differ from each other in the structure and disposition of them. It is familiar to every one that plants contain various substances, as sugar, gum, acids, odoriferous fluids, and others, to which their various flavours and qualities are owing; and a little reflection will satisfy us that such substances must each be lodged in proper cells and vessels to be kept distinct from each other. They are extracted, or secreted, from the common juice of the plam, and called its peculiar or secreted fluids. Various experiments and observations prove also that air exists in the vegetable body, and must likewise be contained in appropriate vessels. Besides these, we know that plants are nourished and invigorated by water, which they readily absorb, and which, by proper tubes or vessels, is quickly conveyed through their stalks and leaves. It is observed, moreover, that all plants, as far as any experiment has been made, contain a common fluid, which at certain seasons of the year is to be obtained in great quantity, and this is proper SAP AND SECRETIONS. 199 ly called the sap. It is really the blood of the plant, by which its whole body is nourished, and from which the peculiar secretions are made. The great motion, called the flowing of the sap, which is to be detected principally in the spring, and slightly in the autumn, is totally different from that constant propulsion of it which is going on in every growing plant. Its facility to run is the first step towards the revival of vegetation from the torpor of winter. Its exciting cause is heat, and the effect of heat is in proportion to the degree of cold to which the plant has been accustomed. The same principle accounts for the occasional flowing of the sap in autumn after a slight frost. Such a premature cold increases the sensibility of the plant to any warmth that may follow, and produces, in a degree, the same state of its constitution as exists after the long and severer cold of winter. The sap in its passage through the leaves and bark becomes quite a new fluid, possessing the peculiar flavour and qualities of the plant, and not only yielding woody matter for the increase of the vegetable body, but furnishing various secreted substances. These are chiefly found in the bark, and often in large and conspicuous vessels, as the turpentine-cells of the Fir tribe. In herbaceous plants, whose stems are only of annual duration, the perennial roots frequently contain these fluids in the most perfect state, nor are they, in such, confined to the bark, but deposited throughout the substance of the root, as in Rhubarb and Gentian. It may be useful to enumerate some of the most distinct secretions of vegetables. Gum or mucilage, a viscid substance of little flavour, exudes from many trees in the form of large drops or lumps, as in Plum, Cherry, and Peach trees. Resin is a substance soluble in spirits, and it differs according to the peculiar tree from which it is obtained. The more refined and volatile secretions of a resinous nature are called essential oils, and they are often highly aromatic and odoriferous. They exist in the highest perfection in the perfumed effluvia of flowers, some of which, capable of combination with spirituous fluids, are obtainable by distillation, as that of the Lavender and Rose. The bitter secretion of many plants does not seem exactly to accord with any of the foregoing. Some facts would seem to prove it of a resinous nature, but it is often perfectly 200 PROCESS OF VEGETATION. soluble in water like gum or mucilage. Acid secretions are well known to be very general in plants. The astringent principle would seem to be a sort of acid, of which there are many different forms, or kinds, and among them the tanning principle of the Oak, Willow, and others. To the secretion of plants we owe the existence of sugar. In tropical countries it is commonly obtained from the expressed juice of the sugar-cane, but the Maple of the North yields it equally pure and scarcely less abundant. It exists also in the roots of some, and in the esculent fruit of many plants, communicating a sweet and usually an agreeable taste. To the foregoing secretions of vegetables may be added those on which their various colours depend. We can but imperfectly account for the green so universal in their herbage, but we may gratefully acknowledge the beneficence of the Creator in clothing the earth with a colour the most pleasing and the least fatiguing to our eyes. We may be dazzled with the brilliancy of a flower-garden, but we repose at leisure on the verdure of a grove or meadow. QUESTIONS.-1. What is said of the whole vegetable body? 2. What are called the peculiar or secreted fluids of plants? 3. What is said of the sap? 4. The flowing of the sap? 5. What are some of the most distinct secretions of vegetables? 6. What is said of those secretions on which the colours of vegetables depend? LESSON 90. Process of Vegetation. Incipient, just beginning. Suc'culent, juicy, moist. WHEN a seed is committed to the ground, it swells by the moisture which its vessels soon absorb, and which, in conjunction with some degree of heat, stimulates its vital principle. Atmospherical air is also necessary to incipient vegetation, for seeds in general will not grow under water, except those of aquatic plants, nor under an exhausted receiver. Seeds buried in the ground to a greater depth than is natural to them, do not vegetate, but they often retain the power of vegetation for an unlimited period. Earth taken from a considerable depth will, when exposed to the air, be PROCESS OF VEGETATION. 201 soon covered with young plants, though no seeds have been allowed to have access to it. The young root is the first part of the infant plant that comes forth, and by an unerring law of nature, it is sent downwards, to seek out nourishment as well as to fix the plant to the ground. In sea-weeds, it seems merely to answer the latter purpose. In the Dodder, the original root lasts only till the stems have established themselves on some vegetable, on whose juices they feed by means of other roots or fibres, and then it withers away. When the young root has made some progress, the two lobes, commonly of a hemispherical figure, which compose the chief bulk of the seed, swell and expand, and are raised out of the ground by the ascending stem. These lobes are called the Cotyle'dons, and between them is seated the Embryo, or germ of the plant. The leaves of the germ being of a succulent nature, assist the plant by attracting from the atmosphere such particles as the tender vessels are fitted to convey. These particles, however, have not in their own nature a sufficiency of nutriment for the increasing plant. The substance or farina of the lobes becomes soft and sweet, being converted into sugar, and is conveyed as long as it lasts to the tender plant, by means of innumerable small vessels, which are spread through the lobes; and which, uniting into one common trunk, enter the body of the germ, and thus supply that balmy liquor, without which the plant must inevitably have perished; its root being then too small to absorb a sufficiency of food, and its body too weak to assimilate it into nourishment. Such is the general course of vegetation in plants furnished with two lobes or cotyledons. But there is a very distinct tribe, which have but one lobe, and are called monocotyle'dons. These are the grass and grain tribe, and many others, in which the body of the seed does not ascend out of the ground. The preservation of the vital principle in seeds is one of those wonders of nature which pass unregarded, from being every day under our notice. Some may be sent round the world through every vicissitude of climate, or be buried for ages deep in the ground, and yet, in favourable circumstances, they will vegetate. Others in order to succeed must sow themselves, in their own way, and at their own time. Great degrees of heat, short of boiling, do not impair their vegetative power, nor do we know any degree of cold that has such an effect. Those who convey seeds from dis tant countries, should be instructed to keep them dry; for if they receive any damp sufficient to cause an attempt at vege tation, they necessarily die, because the process cannot, as they are situated, go on. It is usual with gardeners to keep melon and cucumber seeds for a few years, in order that the future plants may grow less luxuriantly, and be more abundant in blossoms and fruit. Dr. Darwin accounts for this from the damage which the lobes may receive from keeping, by which their power of nourishing the infant plant, at its first germination, is lessened, and it becomes stinted and dwarfish through its whole duration. QUESTIONS.-1. What takes place when a seed is committed to the ground? 2. What is said of the young root? 3. Of sea-weeds? 4. Of Dodder? 5. What are the two lobes called? 6. The germ? 7. How do the leaves of the germ assist the plant? 8. To what use is the farina of the lobes applied? 9. What are plants called that have only one lobe? 10. What is said of the preservation of the vital principle in seeds? 11. Why do gardeners sometimes keep melon and cucumber seeds for a few years? 12. How does Dr. Darwin account for this? LESSON 91. Roots, Stems, Buds, and Leaves. Rad'icle, the minute branch of a root. Physiology, the doctrine of the constitution of the works of nature. Perspire', to give out moisture. Absorb', to take in moisture. THE root of a plant consists of two parts, the body of the root, and the fibre. The latter only is essential, being the part which imbibes nourishment. Roots are either of annual, biennial, or perennial duration. The first belong to plants which live only one year, or rather one summer, as barley; the second to such as are produced one season, and, living through the ensuing winter, produce flowers and fruit the following summer, as winter-rye and wheat; and the third to those which live and blossom through many succeeding seasons to an indefinite period, as trees and many herbaceous plants. Botanists distinguish several different kinds of roots, which are necessary to be known, not only for botanical purposes, but as being of great importance in agriculture and |