Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/480

 434

angiosperms

anther itself betwixt adjacent sporangia, so that their cavities ment usually in the outer if there be two. These inunite. In the ripe anther, therefore, the traces of the teguments are then ovular (sporangial) outgrowths which original sporangia are somewhat obliterated. The spores primarily construct the normal channel of entrance of the are produced in tetrads, each within a sporocyte, by two pollen-tube, and permit of the elaboration of the watersuccessive bipartitions of its energid. In monocoty- carrying system of the ovule, but, retaining more or less ledonous plants each partition is completed by formation their meristematic character, they subsequently, when sharof a cell-membrane around the daughter-energids, whilst ing in the formation of the seed-coat, alter their structure in dicotyledonous plants the partition of the mother- and may form arils of kinds. In holoparasites which form energids into four daughter-energids is effected before these ovules the integument is often absent. In most gamopetare invested by cell-membrane—a hastening of develop- alous Dicotyledones there is only one, but in Monocotylement observable also elsewhere. The spores when mature dones and most polypetalous Dicotyledones there are two. commonly become separated one from the other, but may The significance of this is not apparent. That the integuremain united in tetrads, which may themselves remain ments are structures of slight phyletic import is shown not coherent. Even when isolated the spores do not always only by their somewhat varied structure and points of form a dry powder, but may be entangled by connecting origin, but by the fact that in some families, for instance, threads of the debris of the sporangial and antherine tissue. the Ranunculacese and the Rosacese, genera are unitegOf various shapes, the mature spores have the usual wall minous, whilst others are bitegminous. As the nucellus of two layers, here called the exine and intine. The develops, spore-formation proceeds within it. The sporoexine, more or less cuticularized, may be smooth, but more genous tissue is usually a short sub-apical hypodermal frequently exhibits a pattern through local thickenings, axile row of cells, one of which becomes the megaspore or and there are usually small areas where ,it is thin and less embryo-sac. There are in some cases several embryo-sacs cuticularized, the so-called germ-'pores, through which the which may develop, up to a certain point, when one takes intine, which is a delicate cellulosic membrane, will pro- the lead and the others are absorbed by it. The embryotrude in the later germination of the spore. All these con- sac begins to enlarge at a very early period of the growth ditions of shape, pattern, freedom, coherence of the spores, of the nucellus and elongates in the axis of this and towards are of infinite variety ; in this respect they markedly con- its base, absorbing the micellar tissue. It also enlarges trast with the spore-features in the Pteridophytes, and laterally by the same method. As its enlargement proceeds, find their explanation in the work of pollination. Whilst so do changes in its energid, which divides in the transstill within the pollen-sac the spore reaches a stage of verse plane of the sac. The daughter-energids recede to development at which its germination can begin. Its the respective poles of the elongated sac, and there each energid divides, but the daughter-energids are not of equal divides by two successive bipartitions to form a group of dimensions, and neither of them in Dicotyledones becomes four. Of the basal polar group three invest themselves invested by cell-membrane ; in Monocotyledones this may with cell-membrane and constitute the antipodal cells of the be formed. One, the larger, has a purely nutritive destiny, mature embryo-sac, the fourth moves upwards towards the and is termed therefore the vegetative energid; the other, equator of the sac. Of the apical polar group three remain smaller, is the mother of the male gametes, and is termed at the apex, but do not invest themselves with cellthe generative energid. In many dicotyledonous plants membrane, and constitute the so-called egg-apparatus, one this generative energid divides in the spore to form two of them larger than the others being the female gamete or daughter-energids, each of which is a functional male egg, the others the synergidce or help-cells; the fourth, gamete, and the spore therefore within the pollen-sac which is the sister of the egg, moves downwards towards contains three energids. In monocotyledonous plants this the equator of the sac, and meets the detached fourth further division of the generative energid may be postponed member of the basal polar group, with which it sooner or to a later phase of germination. These changes within the later coalesces to form the definite energid of the embryopollen-grain present us with the reduced homologues of sac. Viewing the embryo-sac as a megaspore, an obvious the male gametophyte and its products in the Pteridophytes, interpretation of these changes within it is that they are and the reduction is much greater than in Gymnosperms. the consequence of its germination, and are therefore the The pollen-grain in this condition is ready for pollination, homologues of the endospermic prothallus of Gymnosperms and unless this occurs no further change normally takes and of the female gametophyte in the Pteridophytes; this place in it, although if the pollen-grains within the anther explanation has received very general acceptance. The be wetted, say by rain, one or more futile pollen-tubes may antipodal cells would be cells of a reduced prothallus; of be sent out. It is to guard against this that so many the three energids of the egg-apparatus, one alone is protective arrangements to the pollen are provided in the normally functional as the egg, but the synergidae may, it has been said, function as eggs. The crux is, however, flower. The destiny of the ovule as the nursery of the embryo in the interpretation of the definite energid, and its formaSpermophytes imposes the necessity of a more elaborate tion has been explained as a case of reinvigoration of a construction than is found in megasporangia decadent vegetative organ by coalescence similar to what an^fum°r" unconnected. with seed - formation, and this is frequently observed elsewhere in the plant kingdom, finds expression in its envelopes and the water- notably amongst the Fungi. It would be then an element carrying system and food-store they provide. The ovule of the prothallus. Other interpretations have traced a arises as a cellular papilla upon the placenta, and as sexual act, and a conjugation of male and female gametes it gradually acquires its mature form, from atropous to in the formation of the definite energid, or a concrescence of anatropous, and with or without a conspicuous funicle, its two female gametes. It is, however, unnecessary to discuss nucellus becomes invested by one or two envelopes, the these, because recent researches have brought to notice integuments, which, growing out from beneath its base, altogether new facts in the history of the changes in the form, by their upward extension, a micropyle at its apex. embryo-sac in fertilization, and to them reference will The vascular bundle-system of the placenta is prolonged presently be made; these, though they do not clear up into the base of the nucellus, but future micellar develop- the difficulties of interpretation of the formation of the ment precludes its further extension therein; it is there- definite energid, must be taken as a starting-point in any fore arrested expanding to a varying degree, or it explanation that may be advanced. The embryo-sac in spreads with often intricate ramifications in the integu- most ovules has arrived at the stage of development above