This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
[We have already given the highest commendation in onr power to Professor Asa Gray's most luminous rudimentary work on botany, just issued by Ivison & Phinney, New York, the title of which is First Lessons in Botany and Vegetable Physiology; but being anxious to enlist in its favor the whole of our readers, and through their influence to endeavor to have it introduced into schools and colleges, we applied to the author for permission to copy one of the chapters. For that purpose we selected the Twenty-Second Lesson (" How Plants Grow"), and present the necessary cuts to illustrate this interesting topic, the theory of which is so lately adopted by the scientific world as to be taught only in books of the last quarter of a century].
" 380. A plant grows from the seed, and from a tiny embryo like that of the Maple (Fig. 1), becomes perhaps a large tree, producing every year a crop of seeds, to grow in their turn in the same way. But how does the plant grow? A little seedling, weighing only two or three grains, often doubles its weight every week of its early growth, and in time may develop into a huge bulk, of many tons' weight of vegetable matter. How is this done? What is vegetable matter? Where did it all come from? _ And by what means is it increased and accumulated in plants? Such questions as these will now naturally arise in any inquiring mind; and we must try to answer them.
"381. Growth is the increase of a living thing in size and substance. It appears so natural to us that plants and animals should grow, that people rarely think of it as requiring any explanation. They say that a thing is so because it grew so. Still, we wish to know how the growth takes place.
"382. Now, in the foregoing Lessons, we explained the whole structure of the plant, with all its organs, by beginning with the seedling plantlet, and following it onward in its development through the whole course of vegetation. So, in attempting to learn how this growth took place, it will be best to adopt the same plan, and to commence with the commencement - that is, with the first formation of a plant. This may seem not so easy, because we have to begin with parts too small to be seen without a good microscope, and requiring much skill to dissect and exhibit. But it is by no means difficult to describe them; and with the aid of a few figures, we may hope to make the whole matter clear.
"383. The embryo in the ripe seed, is already a plant in miniature, as we have learned in the Second, Third, and Twenty-First Lessons. It is already provided with stem and leaves. To learn how the plant began, therefore, we must go back to an earlier period still; namely, to the formation and:
" 384. Growth OF the Embryo itself. For this purpose, we examine the ovule in the pistil of the flower. During, or soon after blossoming, a cavity appears in the kernel or nucleus of the ovule, lined with a delicate membrane, and so forming a closed sac, named the embryo-sac. In this sac or cavity, at its upper end (viz. at the end next the orifice of the ovule), appears a roundish little vesicle or bladder-like body, perhaps less than one thousandth of an inch in diameter. This is the embryo, or rudimentary new plant, at its very beginning. But this vesicle never becomes anything more than a grain of soft pulp, unless the ovule has been acted upon by the pollen.
Germinating embryo of a maple.
"385. The pollen which rails upon the stigma, grows there in a peculiar way: its delicate inner coat extends into a tube (the pollen-tube), which sinks into the loose tissue of the stigma and the interior of the style, something as the root of a seedling sinks into the loose soil, reaches the cavity of the ovary, and at length penetrates the orifices of an ovnle. The point of the pollen-tube reaches the surface of the embryo-sac, and in some unexplained way causes a particle of soft, pulpy, or mucilaginous matter (Fig. 2) to form a membranous coat, and to expand into a vesicle, which is the germ of the embryo.
"386. This vesicle (shown detached and more magnified in Fig. 3) is a specimen of what botanists call a cell. Its wall of very delicate membrane incloses a mucilaginous liquid, in which there are often some minute grains, and commonly a larger soft mass (called its nucleus).
Flg. 2. - Magnified pistil of Back wheat; the ovary and ovnle divided lengthwise: some pollen on the stigmas, one grain distinctly showing its tube, which penetrates the style, reappears in the cavity of the ovary, enters the month of the ovnle (o), and reaches the surface of the embryo-sac (s), near the embryonal vesicle (v).
Fig. 3. - Vesicle or first cell of the embryo, with a portion of the summit of the embryo-sac, detached.
Fig. 4. - Same, more advanced, divided into two cells.
Fig. 5. - Same, a little farther advanced, consisting of three cells.
Fig. 6 - Same, still more advanced, consisting of a little mass of young cells.
Fig. 7. - Forming embryo of Buckwheat, moderately magnified, showing a nick at the end where the cotyledons are to be.
Fig. 8. - Same, more advanced in growth.
Fig. 9. - Same, still farther advanced.
Fig. 10. - The completed embryo, displayed and straightened ont; the same as shown in a section when folded together.
"387. Growth takes place by this vesicle or cell, after enlarging to a certain size, dividing by the formation of a cross partition into two such cells, cohering together (Fig. 4); one of these into two more (Fig. 5); and these repeating the process by partitions formed in both directions (Fig. 6); forming a cluster or mass of cells, essentially like the first, and all proceeding from it After increasing in number for some time in this way, and by a continuation of the same process, the embryo begins to shape itself; the upper end forms the radicle or root-end, while the other end shows a notch between two lobes (Fig. 7); these lobes become the cotyledons or seed-leavee, and the embryo, as it exists in the seed, is at length completed (Fig. 10).