751. Capacity of the cell. Such being the vital energy of the cell, it is easy to admit the possibility of either its solitary existence as a plant (Protococcus, etc.), or of its associated existence, as in the living tissue of most plants.
752. Two modes of cell-growth. Now all plants, without exception, do actually commence existence in the state of a simple cell. But while in the lower plants (Cryptogamia), this simple cell, the plant-rudiment is at once discharged, free and independent, to float or grow, in the Phaenogamia it is yet a while protected and nourished by other cells, - the cells of the ovule.
753. A distinction. This primitive cell-plant, after acquiring the requisite means, swells and divides itself into two or more new cells. If these new cells cohere into a tissue assuming a definite form, as in the higher plants, the process is called growth; but if they separate, each one still abiding separate, it is reproduction.
754. The embryonic vesicle is the expressive name of the embryonic cell of the Flowering Plants- It has its birth in that large cell of the nucleus of the ovule (§ 539) called the embryo sac, and is in some way developed from the cytoblast (§ 655). In appearance it may be like other new cells, but in the impulse or instinct with which it is endowed it is immeasurably different. It looks not to the mere continuation of an old series, but is the projector and pioneer of a new.
755. Its new impulse. Before it can enter upon its course of development so different from the destination of common cells, it must somehow be quickened and energized with an impulse in this new direction. In other words, it must be fertilized, - a process dependent on the pollen grains
756. The pollen tube - its course. When the pollen falls upon the stigma, it imbibes the saccharine moisture there, expands, and its inner, expansible coat of protoplasm protrudes through the aperture (one or more) of the outer crustaceous coat, in the form of an attenuated tube. This, like a radicle, sinks into the soft tissues of the stigma and style, reaches the ovary, and there meets and enters the micropyle of the ovule.
757. Its contents, how discharged. At this juncture the ovule has so turned itself, whether orthotropous, anatropous, etc., as to present the micropyle favorable to this process. The pollen tube makes its way finally to the nucleus and penetrates to the embryo sac. Here its growth ceases; its point is applied externally to the sac, sometimes indents it; but (according to the most accurate observations), does not penetrate it. During this contact the contents of the tube pass by absorption into the sac.
758. Growth of the fertilized cells. Immediately the embryonic globule, thus, somehow endowed with a new instinct, now
607, Section of the ovary of Polygonum Pennsylvanicum, in process of fertilization. (Magnified 20 diameters). c, Natural size n, One of the stamens having discharged its pollen, t, A grain of pollen and its tube. 8, Styles and stigmas. o, Ovary, ovule, embryo sac containing the embryonic globule. The extremity of a pollen tube is seen in contact with the embryo sac first expands into a proper cell, and is usually attached to the wall of the sac near the micropyle. It then divides itself transversely, becoming two cells; the upper elongates either with or without subdivision, forming a filament (suspensor); the lower cell enlarges by subdivision, first spherically, and afterwards the little mass begins to take form according to the species, showing cotyledons, plumule, etc., until fully developed into the embryo.
608, Growth of the embryo in Hippuris vulgaris. The fertilized cell has divided itself into several, of which c, b, constitute the suspensor attached to the apex of the sac; a, embryo dividing into 2, then into 4 cells.
759. Schleiden's view. Owing to the extreme difficulty of observation in this minute field, different views of this process have been advanced. That of Schleiden should not be overlooked. He maintains that the end of the pollen tube actually penetrates the sac and itself becomes the embryonic cell. The pollen grain is in this view the primitive cell, and is itself quickened into development by the contents of the embryo sac.
760. Fertilization in the coniferae. Where no style or stigma exists, as in the Coni-ferae, the pollen falls directly into the micropyle of the naked ovule and its tubes settle into the tissue of the nucleus.
761. Chemical changes in germination. The ovule matures with the completion of the embryo, and passes into the fixed state of the seed in which the embryo sleeps. A store of nutritive matter, starch, gluten, etc., is thoughtfully provided in the seed for the use of the young plant in germination, until its root has gained fast hold of the soil.
762. The changes which occur in the seed at the recommencement of growth, are simply such as are requisite to reduce its dry, insoluble deposits to a solution which shall contain the proper materials for cell-formation or growth; that is, gluten and other nitrogenous matters, oil, starch, etc., are to be changed to diastase, the same as yeast, and dextrine, the same as gum or grape sugar.
609, Ovule of Viola tricolor, showing the process of fertilization according to the views of Schleiden. p, Pollen; t, tube, r, raphe; c, chalaza; b, primine; a, secundine; n, nucleus; 8, sac which the tube appears to have penetrated.
763. The process. To this end water and oxygen are absorbed, the gluten begins decomposition, forming yeast; fermentation ensues; heat is produced by the slow combustion of the carbon with oxygen forming and evolving carbonic acid, by which process some of the oil and starch is destroyed, while another portion gains water and turns to sugar. All this within the cells of the seed.