200 PROCESS OF VEGETATION. soluble in water like gum or mucilage. Acid secretions are well known to be very general in plants. gent 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. Incip'ient, 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 monocoty le'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 distant countries, should be instructed to keep them dry; for if they receive any damp sufficient to cause an attempt at vegetation, 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 gardening. Barren and thin soils are best suited to the wide spreading roots, which creep extensively on the surface; dry and sandy plains are adapted to those which penetrate deep for nourishment, and are supplied with bulbs for its preservation, or with downy radicles for its abundant absorption. Linnæus enumerates seven kinds of trunks, stems, or stalks of vegetables. These are necessary to be known for botanical distinctions, though some are more important than others. About midsummer the progress of vegetation seems to be suspended, and for several days the vital energies of the tree are exerted in the formation of buds. We no longer observe the vigorous growth of spring, but if we examine the young branches, we shall find the newly formed buds at the base of the leaf-stalk, immediately above the place of their insertion. After the fall of the leaves they are more conspicuous, and during the winter we may perceive a gradual enlargement, corresponding to the developement of the tender germs which they enclose. Plants, as is well known, may be propagated by buds, and in that sense each bud is a separate being, or a young plant in itself; but such propagation is only the extension of an individual, and not a re-production of the species, as by seed. Leaves are eminently ornamental to plants from their pleas ing colour, and the infinite variety as well as elegance of their forms. Their different situations, insertions, forms, and surfaces, which are of the greatest possible use in systematical botany, cannot here be described. A knowledge of their real use with regard to the plant is a curious branch of vegetable physiology. That leaves give out moisture, or are organs of insensible perspiration, is proved by the simple experiment of gathering the leafy branch of a tree, and immediately stopping the wound at its base with wax to prevent the effusion of moisture in that direction. In a very short time the leaves droop, wither, and are dried up. If the same branch, partly faded, though not dead, be placed in a very damp cellar, or immersed in water, the leaves revive, by which their power of absorption is also proved. A know ledge of the perspiring and absorbing power of leaves is often of great practical importance. It teaches us that plants droop, in consequence of the excess of the former, and are to be revived by diminishing their discharge, or increasing their absorption. The former is accomplished by confining the air around them, and the latter by sprinkling water over the leaves; and when plants have recently been removed, such management is frequently required. Air is not less essential to the healthy existence of animals than of plants. One great use of leaves is to perform, in some measure, the same office for the support of vegetable life, that the lungs of animals do for the support of animal life. Light has a very powerful effect upon plants, and the green colour of leaves is so much owing to it, that plants raised in darkness are of a sickly white Light acts beneficially upon the upper surface of leaves, and hurtfully upon the under side; hence the former is always turned towards the light, in whatever situation the plant may be placed. A great number of leaves follow the sun in its course, and a familiar instance of this is a clover-field. The leaves of some plants, when the light is withdrawn, fold over each other, or droop as if dying; and this is called by Linnæus the sleep of plants. Some leaves display an extraordinary sensibility to the touch of any extraneous body, or to any sudden concussion, as those of the sensitive plant. An impression made, in the most gentle manner, upon one of its leaflets, is communicated in succession to all of them, evincing an exquisite irritability. The moving plant of India exhibits such powers as to excite the astonishment of every beholder. If its motion, be impeded, no sooner does it regain its liberty than its operations are renewed with increased activity, as if it were necessary to redeem the time which it had lost. Its winged leaves seem to disdain to rest, and to exhibit a most astonishing example of industry. QUESTIONS.-1. What are the two parts of the root of a plant? 2. How are roots divided with regard to their duration? 3. Give the examples. 4. What is said of buds? 5. How is it proved that leaves are organs of perspiration, and of absorption? 6. What office do leaves perform for plants? 7. What is the effect of light upon plants, and leaves? 8. What is said of the sensitive plant? 9. Of the moving plant of India? 10. Describe the several kinds of Roots, (see Appendix.) 11. What is said of the root of common herds grass? 12. What are the seven kinds of trunks or stems? 13. What are the several kinds of appendages to a plant? 14. What are the several kinds of Inflorescence? |