The aphids or plant-lice reproduce during the summer by parthenogenesis, but in the autumn winged males and females appear, and fertilized winter eggs are produced. From these eggs there develop, in the following spring, the wingless parthenogenetic summer forms, which produce the successive generations of the wingless forms. As many as fourteen summer broods may be produced. By keeping the aphids in a warm temperature and supplying them with plenty of moist food, it has been possible to continue the parthenogenetic reproduction of the wingless forms for years. As many as fifty successive broods have been produced in this way. It has not been entirely determined whether it is the temperature or a change in the amount, or kind, of food that causes the appearance of the winged males and females, although it seems fairly certain that diminution in the food, or in the amount of water contained in it, is the chief cause of the change. In the honey-bee the remarkable fact has been well established that fertilized eggs give rise only to females (queens and workers), while unfertilized eggs develop into males. Whether a fertilized egg becomes a queen or a worker (sterile female) depends solely on the kind of food that is given to the young larva, and this is determined, in a sense, entirely by the bees themselves. In plants also there are many cases of parthenogenesis known. Some species of Chara when kept under certain conditions produce only female organs, and seem to produce new plants parthenogenetically. In this case it appears that the same conditions that caused the plants to produce only female organs may also lead to the development of the egg-cells without fertilization. In fact it is only by a combination of this kind that parthenogenesis could arise. The result is similar when the eggs of insects produce only females whose eggs are capable of parthenogenetic development. If a case should arise in which only females appeared whose eggs did not possess the power of parthenogenetic development, the species would die out. In the green alga, Spirogyra, it has been found that if conjugation of two cells is prevented, a single cell may become a parthenogenetic cell. In a number of parasitic fungi the male organs appear to be degenerate, and from the female organs parthenogenetic development takes place. A small number of flowering plants are also capable of parthenogenetic reproduction. There is a peculiarity in the development of the parthenogenetic eggs of animals that will be more fully discussed later, but may be mentioned here. Ordinarily an egg that becomes fertilized gives off two polar bodies, but in a number of cases in which parthenogenetic development occurs it has been found that only one polar body is given off. It is supposed that in such cases one polar body is retained, and that it plays the same part as the entrance of the spermatozoon of the male.