CHAPTER XXXI. 



* 



SEX AND HEREDITY. 



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 indivi- 

 dual. 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 Schizophyta are 

 examples of this, while further instances are seen in Euglena and 

 Protococcus. In certain Plants also of advanced organisation syngamy 

 may be absent, as in some Flowering Plants and Ferns ; and notably 

 in the higher Basidiomycetous and Ascomycetous Fungi. There is 

 reason to believe that in such cases syngamy has been omitted or 

 lost, for comparison indicates that all these plants have been descended 

 from a sexually propagated ancestry. Thus syngamy, though pre- 

 valent, is not an indispensable feature in every life-cycle. 



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

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

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

 Such cells are called isogametes t 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 clear 

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

 distinction of sex was later derived. The isogametes themselves 

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

 green and brown Algae : Ulothrix (Fig. 330, p. 392), Acetabularia 

 (Fig. 335, p. 397), and Ectocarpus siliculosus (Fig. 321, p. 383) are 

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

 Conjugatae, such as Spirogyra (Fig. 337, p. 399). 



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