CHAPTER XXXV. 



HEREDITY AND VARIATION 



Sexual Reproduction. 



In the great majority of Plant Organisms certain sexual cells called 

 Gametes are produced, which fuse in pairs. The process is called 

 Syngamy, or Sexual Fusion. The result of it is the production of a 

 single cell, the Zygote, which forms the starting point for a new indi- 

 vidual. Though such syngamy is a very wide-spread fact among 

 living things, whether Animals or Plants, it is not universal. Some 

 primitive organisms are without it. The whole series of the Schizo- 

 phyta are examples of this, while sexuality is rare or doubtful in 

 Euglena and Pleurococcus. In certain Plants also of advanced organisa- 

 tion syngamy may be absent, as in some Flowering Plants and 

 Ferns. 



A comparison of related organisms low in the scale, which show 

 syngamy, suggests that in the first instance the fusing gametes wi 

 alike in size and behaviour, though more or less distinct in their origin. 

 Such cells are called isogametes, and the process of their fusion is de- 

 scribed as conjugation. It is seen in both Animals and Plants of low- 

 organisation. This condition, where the gametes show no char 

 distinction of sex, is believed to represent a primitive state from which 

 distinction of sex was later derived. The isogametes themselves m 

 be motile or non-motile. The former is seen typically in various 

 green and brown Algae : Ulothrix (Fig. 270, p. 365), Acetabular ia 

 (Fig. 275, p. 370), and Ectocarpus siliculosus (Fig. 284, p. 380) are 

 cases in point. Conjugation of non-motile isogametes occurs in the 

 Conjugatae, such as Spirogyra (Fig. 277, p. 373). 



If two organisms, each consisting of only a single cell, fuse to 

 form one, the immediate result is a diminution in number to one 



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