of the blood, even in the coldest climate, being maintained at 96°.

"In regulating the temperature of his body, man follows instinctively the same rules of common sense which he applies in warming his dwellings. In proportion as the climate is cold, he supplies the loss of heat by burning more fuel in his lungs, and hence the statements of arctic voyagers, who have told us that twelve pounds of tallow-candles make only an average meal for an Esquimaux, are not inconsistent with the deductions of science."

143. Slow and Rapid Combustion of Sugar.—We may compare the rapid combustion of sugar in air with its slow combustion in the body, by the following experiment: Take 2 ounces of pulverized sugar, or the average quantity burnt in the body of a man in an hour, and mix it with 5 ounces of pulverized potassic chlorate. We have then the sugar and the solidified oxygen of the chlorate mingled as in the blood; but the oxygen is passive, and will not combine with the sugar, until roused to activity. A single drop of sulphuric acid let fall upon the mixture serves to awaken its dormant energy; and the mass is consumed in an instant, with intense evolution of heat and light. The amount of heat concentrated in this momentary burst of flame is no greater than would have been generated in the blood in the course of an hour.

"The splendid displays of combustion arrest our attention by their very brilliancy, while we overlook the silent yet ceaseless processes of respiration and decay, before which, in importance and magnitude, the greatest conflagrations sink into insignificance. These are but the spasmodic efforts of nature; those, the appointed means by which the harmony and order of creation are preserved."

66

144. The Daily Consumption of Oxygen in Nature. Faraday has roughly estimated that the amount of oxygen required daily, to supply the lungs of the human race, is at least one thousand millions of pounds; that required for the respiration of the lower animals is at least twice as much as this, while the always active processes of decay require certainly no less than four thousand millions of pounds more, making a total aggregate of seven thousand millions of pounds required to carry on these processes of nature alone. Compared with this, the one thousand millions of pounds which, as Faraday estimates, are sufficient to sustain all the artificial fires lighted by man, from the camp-fire of the savage to the roaring blaze of the blast-furnace, or the raging flames of a grand conflagration, seem small indeed.

Amount of Oxygen required Daily.

"How utterly inconceivable are these numbers, which measure the magnitude of nature's processes, - eight thousand millions of pounds of oxygen consumed in a single day! When reduced to tons, the number is equally beyond our grasp; for it corresponds to no less than 3,571,428 tons. If such be the daily requisition of this gas, will not the oxygen of the atmosphere be in time exhausted? It is not difficult to calculate approximately the whole amount of oxygen in the atmosphere. It is equal to about 1,178,158 thousand millions of tons; a supply which, at the present rate of consumption, would last about nine hundred thousand years."

THE GROWTH OF PLANTS.

145. The Embryo Plant in the Seed. How do plants grow? How does the tiny seed become the leafy herb? How does the little acorn develop into the giant oak? We shall get the best answer to these questions by tracing a plant through its whole growth.

The seed is formed in the flower, the essential parts of which are the stamens and the pistil. The stamens bear the anthers, and these contain the pollen. The pistil is in the centre of the flower. It encloses in its ovary the ovules, which, when ripened, become seeds.

In the ripe seed we find the embryo, which is a miniature plant with stem and leaves. How has this embryo been formed?

Fig. 20.

18

At a certain time a little cavity is formed in the centre of the ovule within the pistil. This cavity is called the embryo sac, and is marked s in Figure 20. Within this sac, at its upper end, a we see a minute body or vesicle, v. This is the first germ of the embryo; but in order that it may begin its development, it must be acted upon by the pollen. This we see in the form of small grains, a, resting on the top of the pistil. It has fallen from the anthers, and lodged here; and now it sends out a very fine and delicate tube, the pollen-tube, which pierces the tissue of the pistil, and extends itself until it reaches the vesicle, v. Its contact with this microscopic particle of matter has the mysterious power of making it begin to grow, and thus form the embryo.

The vesicle is at first a single cell; that is, a mem

branous globule, filled with liquid, in which minute particles can sometimes be discerned. After it has been acted upon by the pollen-tube, it enlarges somewhat, and a partition forms across its interior, dividing it into two cells. Each of these grows and divides in like manner, and thus a cluster of cells is formed. After a time the mass begins to take a definite shape. One end becomes the radicle, or the beginning of the root of the plant; and the other divides into two parts, which develop into the cotyledons, or seed-leaves. It is now a perfect embryo, a miniature plant, with root, stem, and leaves, but still shut up in the seed.

146. The Plantlet. - The growth of the plant in the seed, and out of the seed, is essentially the same process. The same division and multiplication of cells continues, and

all the parts of the plant are formed by the clustering or aggregation of these cells. The cells are too minute to be distinguished with the naked eye, but the microscope shows them in every portion of the plant. Figure 21 shows a thin slice of a rootlet, cut crosswise and lengthwise, as it appears when

Fig. 21.

highly magnified. We see that the whole structure is cellular; that is, made up of cells crowded together. A single cell is represented in Figure 22. The natural shape of the cell is spherical; but when cells are clustered or crowded together, they compress one another into the shape in which we see them in these figures. Where they are not thus squeezed, they are generally found to be spheres, more or less perfect. They vary in size, from about the thirtieth to the thousandth of an inch in diameter.

Fig 22.

At first the cells are all alike in shape and in texture; but they are greatly modified in both respects in forming the varied tissues of the plant. They may be lengthened out into tubes, as in the fibres of cotton, which are single cells thus drawn out. The soft, thin membrane which encloses them may become hard and thick, as in the shell of

a walnut or the tough wood of an oak.

They may be closely crowded together, and nearly filled up with solid matter; or they may be loosely interlaced, and form vessels to contain the vegetable juices. Their contents are as varied as their structure, and, seen through the transparent walls, give rise to all the manifold colors of leaf and flower and fruit.

At

Figure 23 (from Wood's "Class-book of Botany") shows a few of the varied forms which cells assume. the top we have wood-cells from the fibre of flax. Below

are represented the many-sided cells of the pith of elder; stellate (star-shaped) cells of the pith of the rush; spherical cells of the houseleek; and wood-cells of the oak. 147. Organic Structure. This cellular structure is the characteristic of organic beings; that is, plants and