FERTILIZATION IN PLANTS. Fer tilization is generally defined as the union of definitely organized male and female elements. This definition is strictly correct for plants in which the uniting elements can be called sperms and eggs; but in many of the lower plants algae and fungi-- the elements which unite are alike in size and appearance, not being differentiated as male and female. Many botan ists use the term conjugation to describe the union of such similar gametes, as the uniting elements are called. Between these cases in which the gametes are alike and those in which they are very different, there are all imaginable gradations. However, in all cases, the essen tial feature is the union of the two nuclei of the gametes, whether the gametes be alike or unlike. It is still quite generally believed that the two nuclei fuse in the resting condition and that the chromatin of the two parents becomes mixed. A thorough cytological study of fertilization has been made in only a few plants, and in most of these the chromatin — at the time of fertiliza tion — is in the form of separate chromosomes. (See CHROMOSOME). There is no that the chromosomes of the male and female, in these cases, are entirely separated and that the first division of the egg takes place without any fusion or even close association of chromo somes. The two nuclei resulting from the first division of the egg pass into the resting condi tion and the chromatin of the male and female parents becomes indistinguishable. During all the rest of the life history no one has yet been able to identify the male and female chromosomes; but there is such strong evidence that the chromosome is an individual organ of the cell, maintaining its identity throughout the life history, that most cytologists believe there is never any real fusion of male and female chromosomes, although there is a very close Contact.
Fertilization serves two purposes: it acts as a stimulus to development and it keeps the characters of the species approximately con stant. As a result of the stimulus, the ferti lized egg divides repeatedly and builds up the embryo. There seems to be a limit to the duration of the stimulus and, when that limit is reached, the organism grows old and dies, but, as it reaches maturity, it produces eggs, which, stimulated by fertilization, begin the process again. That fertilization functions in keeping the species approximately constant is generally admitted. Variation is universal and no two individuals are exactly alike. If an individual varies widely from the type, we generally call it a freak or sport. Such freaks or sports are either incapable of fertilization and conse quently leave no progeny, or they are fertilized by some individual of the species which has not varied much, and the union with a normal indi vidual reduces the aberrant form to the ordi nary plane of the species. A perennial illus tration is the six-fingered man, an obvious freak. Intermarriage with normal individuals for generation after generation finally breeded the six-fingered condition out of the family. Doubtless, plants behave in the same way.
Variations which maintain themselves and stubbornly refuse to be reduced to the type are called mutations (q.v.) It is very probable that many °species" have arisen in this way. Con sult Wilson, E. B., The Cell in Development and Inheritance' ; Mottier, D. M., in Plants' (Carnegie Inst. of Washington. Publication No. 15).