80 PART I. THE MORPHOLOGY OF PLANTS. [ 17- 



17. General Morphology of the Sexual Reproductive 

 Organs. The general morphology of the sexual reproductive 

 organs agrees in many respects with that of the asexual repro- 

 ductive organs. 



In the great majority of plants the sexual reproductive organs 

 give rise to sexual reproductive cells, termed gametes; hence the 

 organs may be generally termed gametangia. In some cases the 

 formation of gametaugia is limited to a certain portion of the 

 body of the gametophy te, which differs more or less from the vege- 

 tative portions of the shoot and may be distinguished as &'gameto- 

 phore. When a part of the body is differentiated as a shoot, the 

 gametophore is part (or the whole) of the shoot. Since, in most 

 plants, the morphological differentiation of the gametophyte is not 

 so complete as that of the sporophyte, it follows that the game- 

 tangia are less frequently developed on leaves than are the 

 sporangia. In those cases in which the gametangia are borne by 

 leaves, the leaves may be termed gametophylls. 



(a) The Gametes. A gamete is a sexual reproductive cell a 

 reproductive cell, that is, which is incapable by itself of giving 

 rise to a new organism ; in this respect it differs from a spore. A 

 spore is, however, formed from the fusion of two gametes of 

 different sexes ; that is, by a sexual process (see p. 69). 



In those of the lower Algae and Fungi in which sexual spore- 

 formation takes place, the gametes produced by the organism are 

 all externally similar; hence these plants are termed isogamous', 

 the sexual process, which consists here in the fusion of two simi- 

 lar gametes, is termed conjugation ; and the spore formed by con- 

 jugation is termed a zi/gospore. 



In all the higher plants, hence termed heterogamous, the gametes 

 are not all alike; but there are two kinds, the male and the female. 

 The male and female gametes may be generally distinguished by 

 their difference in size, the male being the smaller, and by the 

 greater activity of the male gamete in connection with the sexual 

 process which is here termed fertilisation, the male gamete being 

 considered to fertilise the female. 



The gradual transition from isogamy to heterogamy can be most clearly traced 

 in the Algse : special attention is directed to this in the account of the Algae in 

 Part III. 



The gametes of isogamous plants, in those cases in which they are 

 set free from the gametangium and are free-swimming, are well 



