CYTOLOGY 29 



place in a variety of different ways. It is quite common for the simplest 

 plants to multiply by the method of binary fission, in which the parent 

 individual splits into two. In others, a small portion of the plant grows out 

 to form a bud, which eventually enlarges into a new individual like the 

 parent, from which it may be finally separated. :\Iany plants, too, produce 

 small, round cells, often with a thick resistant coat which withstands adverse 

 conditions. These cells are called spores, and under suitable conditions 

 they are detached and develop into new plants. 



In all these methods of reproduction only one individual is necessary 

 for the production of a new generation. Such a type of reproduction is called 

 asexual. More commonly, however, reproduction is preceded by a sexual 

 act, for the purpose of which two individuals or two cells are generally 

 required. The sexual act always includes a fusion of nuclei, which is the 

 essential feature, and generally also the fusion of cells. 



In sexual reproduction special cells are set apart in the body for the 

 production of new individuals. In all but the lowest types these cells are 

 of two kinds, which we call male and female. They are often produced in 

 the same individual, but if not, we may distinguish male and female organisms 

 according to the type of cell produced. These special cells are the sex cells, 

 or gametes. For reproduction to take place it is necessary for two gametes, 

 one from the male and the other from the female, to come together and fuse 

 completely, forming a fusion cell, or zygote. The zygote is the potential 

 new individual. 



The male and female gametes are usually unlike in appearance and 

 behaviour. The female gamete is generally a large cell with much cytoplasm 

 and the nucleus is relatively small. As a result of its size this cell is usually 

 non-motile. Such a body is termed an oosphere. 



The male gamete presents a marked contrast to the oosphere. It is 

 comparatively small, and is usually produced in large numbers. It may 

 consist only of nuclear material and at most it has only a small amount of 

 cytoplasm. In very many plants it is provided with hair-like protoplasmic 

 appendages called flagella, by means of which it can propel itself through 

 water. In this way it is able to swim about in search of the female gamete. 

 On coming into contact with an oosphere it has the power of penetrating 

 into the cell, whereupon its nucleus travels through the cytoplasm and fuses 

 with the nucleus of the oosphere. This process is spoken of as fertilization. 

 The resulting fusion cell or zygote may rest for a time, but it eventually 

 starts to grow, dividing again and again, and building up an embryo or 

 immature individual, which subsequently develops into a new adult form. 



Meiosis or Reduction Division 



The essential feature of the union of two gametes in the process of 

 fertilization is the fusion of their nuclei. Each of these gametes possesses 

 the monoploid or haploid («) number of chromosomes, and hence the 

 zygote will have double the number of chromosomes, that is to say, its 



