ANGIOSPERMS
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from the definite energid of the embryo-sac, and the of the inflorescence may be called into requisition to process of double fertilization above described gives supply the adaptation. Special outgrowths, arils, of the new interest to its development. The segmen- seed-coat are of frequent occurrence. In the feature of Hndosperm. tation 0f tpe fertilized definite energid always fruit and seed by which the distribution of Angiosperms begins before that of the egg, and thus there is timely is effected we have a distinctive character of the class. In preparation for the nursing of the young embryo. If in Gymnosperms we have seeds, and the carpels may become its extension to contain the new formations within it the modified and close around these, as in Finns, during the embryo-sac remain narrow, endosperm formation proceeds process of ripening to form an imitation of a box-like fruit upon the lines of a cell-division, but in wide embryo-sacs which subsequently opening allows the seeds to escape; the endosperm is first of all formed as a colony of energids but there is never in them the closed ovary investing from (coenobium), and only gradually acquires a pluricellular the outset the ovules, and ultimately forming the groundcharacter, the change starting from the sides or ends of work of the fruit. Their fortuitous dissemination does not always bring the embr/o-sac. The function of the endosperm is primarily that of nourishing the embryo, and its basal seeds upon a suitable nidus for germination, the primary position in the embryo-sac places it favourably for the essential of which is a sufficiency of moisture, absorption of plastic material entering the ovule. Its and the duration of vitality of the embryo is a duration varies with the precocity of the embryo. It may point of interest. Some seeds retain vitality seed be wholly absorbed by the progressive growth of the for a period of many years, though there is no embryo within the embryo-sac, or it may persist as a warrant for the popular notion that genuine “mummy definite and more or less conspicuous constituent of the wheat ” will germinate; on the other hand some seeds seed. When it persists as a massive element of the seed lose vitality in little more than a year. Further, the older its nutritive function is usually apparent, for there is the seed the more slow as a general rule will germination accumulated within its cells reserve-food, and according be in starting, but there are notable exceptions. This to the dominant substance it may be starchy, oily, pause, often of so long duration, in the growth of the cellulosic, aleuronic, and so forth. In cases where the embryo between the time of its perfect development embryo has stored reserve-food within itself and thus within the seed and the moment of germination, is one of provided for self-nutrition, such endosperm as remains the remarkable and distinctive features of the life of in the seed may take on other functions, for instance Spermophytes. The aim of germination is the fixing of the embryo in the soil, effected usually by means of the that of water-absorption. As the development of embryo and endosperm proceeds root, which is the first part of the embryo to appear, in within the embryo-sac, its wall enlarges and commonly preparation for the elongation of the epicotyledonary absorbs the substance of the nucellus (which is portion of the shoot, and there is infinite variety in the Fruit and ppewise enlarging) to near its outer limit, and details of the process. In albuminous Dicotyledones the seed ' combines with it and the integument to form the cotyledons act as the absorbents of the reserve-food of the seed-coat; or the whole nucellus and even the integument seed and are commonly brought above ground {epigeous),, may be absorbed. In some plants the nucellus is not thus either withdrawn from the seed-coat or carrying it upon absorbed, but itself becomes a seat of deposit of reserve- them, and then they serve as the first green organs of the food constituting the perisperm which may coexist with plant. The hypocotyl commonly plays the greater part endosperm, or may alone form a food-reserve for the in bringing this about. Exalbuminous Dicotyledones. embryo. Endospermic food - reserve has evident advan- usually store reserve-food in their cotyledons, which may tages over perispermic, and the latter is comparatively in germination remain below ground (hypogeous). In rarely found and only in non-progressive series. Seeds in albuminous Monocotyledones the cotyledon itself, probwhich endosperm or perisperm or both are said to exist ably in consequence of its terminal position, is commonly are commonly called albuminous, those in which neither the agent by which the embryo is thrust out of the seed, is found are termed exalbuminous. These terms, exten- and it may function solely as a feeder, its extremity desively used by systematists, only refer, however, to the veloping as a sucker through which the endosperm is grosser features of the seed, and indicate the more or less absorbed, or it may become the first green organ, the terevident occurrence of a food - reserve; many so-called minal sucker dropping off with the seed-coat when the exalbuminous seeds show to microscopic examination a endosperm is exhausted. Exalbuminous Monocotyledones distinct endosperm which may have other than a nutritive are either hydrophytes or strongly hygrophilous plants function. The character of the seed-coat bears a definite and have often peculiar features in germination. It has been possible here only to depict what appear to relation to that of the fruit which is derived from the be general features of sexual reproduction in Angiosperms, ovary, and it may be from accessory parts as well. Their function is the twofold one of protecting the embryo and and these after all rest upon observation within only a of aiding in dissemination; they may also directly pro- limited circle of forms. A new chapter in the history of mote germination. If the fruit is a dehiscent one and the our knowledge of the changes in the embryo-sac is opened seed is therefore soon exposed, the seed-coat has to provide by the discovery of double fertilization, and the whole requires reinvestigation. for the protection of the embryo and may also have to subject Distribution by seed appears to satisfy so well the secure dissemination. On the other hand indehiscent fruits discharge these functions for the embryo, and the requirements of geophilous Angiosperms that distribution Agamoseed-coat is only slightly developed. Dissemination is by vegetative buds is only an occasional process. genetic effected by the agency of water, of air, of At the same time every bud on a shoot has the Dissemina- animals_and fruits and seeds are therefore capacity to form a new plant if placed in suit- ^ctioa. U a ° ' grouped in respect of this as hydrophilous, able conditions, as the horticultural practice of anemophilous, and zooidiophilous. The needs for these propagation by cuttings shows; in nature we see plants are obvious—buoyancy in water and resistance to wetting spreading by the rooting of their shoots, and buds we for the first, some form of parachute for the second, and know may be freely formed not only on stems but on some attaching mechanism or attractive structure for the leaves and on roots. Where detachable buds are prothird. The methods in which these are provided are of duced, which can be transported through the air to a infinite variety, and any and every part of ffhe flower and distance, each of them is an incipient shoot which may
