There are many degrees of cross-fertilization. The cross may take place between two flowers in the same cluster or between two clusters upon the same plant; or it may take place between distinct plants, either of the same or of another species. Fertilization between flowers on the same plant is known as individual-fertilization. The limits within which crossing is possible are not known, but the closer the species are related the more readily, as a rule, will they cross. One of the barriers which nature erects to prevent self or close-fertilization is a difference in time of maturing of the two sexes. In any flower the two parts are rarely ready at the same time. Flowers in which the stamens mature first are said to be proterandrous, and those in which the pistils mature first are proterogynous. In crossing such species, flowers of different ages can usually be found so that the parts can be brought together without difficulty. But when one series of organs in all the flowers of any species perish before the other series is mature, the pollen must be kept until the pistils are ready, or one sex must be forced or retarded artificially to accommodate the other. If the pollen matures first, it is only necessary to keep it a few days until the pistil is ready; but if the pistil matures first, and the plants cannot be handled artificially, the pollen must be kept over until the following season in order to effect any crosses. The longevity of pollen is little understood. That of some species will keep much longer than others. It is supposed that, as a rule, it will not keep beyond a few days or weeks. If the pollen is to be kept, the anthers should be picked just before ready to burst and laid upon paper in a warm, dry and shady place until they dry up and the pollen is all discharged. The anthers must then be removed, and the pollen is securely wrapped in dry paper. If it is to be kept long it will probably be better to place it in small, closely cork-stoppered vials. It should be kept in a uniform temperature. =Methods.=--All perfect flowers--those which contain both stamens and pistils--must be deprived of their anthers before the pollen is discharged, to prevent self-fertilization. This removal of the anthers is called _emasculation_. It is performed before the flower opens, and therefore before any foreign pollen could have reached the stigma. In some flowers, as in the tomato, Fig. 95, the stigma protrudes even before the petals are fully grown and emasculation must be performed very early. Even if the stigma is not mature, there is a chance that pollen will adhere to it and persist until conditions are fit for its growth. The flower is generally emasculated by pulling out the anthers with pincers, but some large anthers can be hooked out easily by a very small crochet-hook or by a pin bent to a minute hook upon the point. [Illustration: Fig. 95. Tomato-flower.]