FERTILIZATION (from Lat. fertilis, fertile, from term. to bear, (;k. Oeptly, pherein, Skt. latnr, Cloth. bairan, OHO., AS. beran, Eng. bear). In plants, the fusion of two sexual cells or gametes (q.v.). This process, sometimes called 'fecunda tion,' is exhibited in the life histories of t plants. Gametes are present in all plants, except ing the tower forms. The possible derivation et gametes has been suggested by the life histories of certain alga• as Clothrix, in which there is an evident relationship between gametes and the ordinary asexual swimming-spores. It is prob able that gametes have in general been derived from asexual spores, and it is not surprising that they occasionally germinate as asexual spores. In the simplest C ,ce, of fertilization the gametes are so that there is no distinc tion of sex. In this case the process is often called 'conjugation,' and plants which exhibit it are known as isoganions' plants. Almost all plants, however, are `heterogamous;' that is, the pairing gametes have become so different that a distinction of sex is plainly evident. In heterog amous plants the male gamete is known as the 'sperm,' and the female gamete as the 'egg.' and fertilization in its restricted sense is the fusion of sperm and egg. Along with the differentiation of gametes has occurred the differentiation of sex organs (gametangia), those developing sperms being called `antheridia,' those developing eggs being called 'ot;gonia' or 'archegonia.' The result of fertilization is the formation of a spore which is in all plants technically the 'oi)spore,' or fer tilized egg. To distinguish the result of conjuga tion from that of ordinary fertilization, the name 'zygospore' or 'zygote' is commonly substituted for 'otispore.' Among seed-plants a single free egg is developed within the embro-sae, and with in the pollen-grain certain cells are developed which are called 'male cells,' and which are sperms in function.
One of the chief features in connection with the process of fertilization is the method by which the pairing gametes are brought together. In conjugation the two gametes have similar powers, and in the majority of eases are motile hodies, usually swimming by means of cilia. in sonic cases in the lower forms the gametes are brought together by the development of a tube which bridges the space between the sex-organs. In heterogamous plants the sperm is always the active gamete, and finds its way to the egg. which remains quiescent. Among the higher akqe, masses, and ferns the sperm is, as a rule, a free swimming ciliated cell, and is attracted to the obganimn or archegonium by various chemical substances which are secreted in connection with the female sex-organ. One of the distinguishing marks of the seed-plants, however. is that the sperm ceases to lie a motile body, and is ordina rily transferred from the pullen-grain in which it is produced to the egg by means of a tube called the 'pollen-tulic.' This tube in gymnosperms
(pines, etc.) penetrates the tissues of the ovule lying above the embryo-sac, and in the angio sperms (flowering plants) it passes through the style, enters the ovary, and finds its way to tile ovule. It is an interesting fact that in the eyeadaceic (q.v.) ciliated sperms have recently been discovered, which indicates that the old free-swimming habit of the sperm had not been entirely abandoned with the introduction of pol len-tubes.
While the passage of the sperm to the egg may be included under fertilization, the real net of fertilization consists in the fusion of the two naked cells. dust what happens in this fusion may be illustrated by the process of fertilization in the seed-plants. Each gamete is a naked cell consisting of a nucleus about which there is organized cytoplasm. In the sperm the nucleus is very prominent, and the cytoplasm relatively scanty. In the egg the nucleus is also prominent, but the cytoplasm is abundant, and contains a rich snpply of food reserve. In the seed-plants it has been observed that the pollen-tube approaches the egg and discharges a male cell into the cyto plasm. The nucleus of the male cell then slips out of its cytoplasm as out of a sheath, and moves through the cytoplasm of the egg toward its nucleus. The male has thus been left behind in the egg-cytoplasm, and it is not clear that the fusion of the two holds any im portant relation to the act of fertilization. In any event, the most evident fact in fertilization is the approach of the male nucleus to the female nucleus, and the fusion of the two. Just what this fusion involves, and how complete it is. is not clear. The nuclei are exceedingly complex structures, and just how far the corresponding structures of the two nuclei fuse in this process is very uneerta in. In certain seed-plants. as in gymnosperms, it has been observed that the two sets of chromosomes, which are thus brought to gether by the fusion of the two nuclei. do not fuse for some time, and in cone cases for several cell-generations. It is evident, therefore, that the fusion does not necessarily involve a fusion of the chromosomes of the nuclei, which are re garded as probably the essential structures. The whole subject remains somewhat vague as to de tails, but the general fact that fertilization in volves the fusion of two cells, and that in this fusion the two cytoplasms and the two nuclei take part, is well made out. The significance of the male cytoplasm, whether it simply acts as a carrier of the nucleus or is an essential feature in the fusion, and the details of nuclear fusion, whether it is a complete pairing of all of the structures which belong to the nuclei or not, are questions which investigation has not yet set tled. See CELL; GA.NIETES ; and articles on the various groups of plants mentioned above.