106 OUTLINES OF EVOLUTIONARY BIOLOGY 



reduced in size and never become free from the spores ; they 

 nevertheless develop antheridia and archegonia respectively, 

 in which spermatozoa and ova are produced, and from the 

 conjugation of these arise zygotes or fertilized ova which 

 develop into new sporophytes. 



We have briefly noticed these heterosporous ferns because, 

 as regards the sexual phenomena which they exhibit, they 

 constitute a very interesting connecting link between the ordinary 

 (homosporous) ferns, which produce only one kind of spore, and the 

 highest members of the vegetable series, the flowering plants. 



In the flowering plants an alternation of sexual and asexual 

 generations can still be traced, but here the gametophyte is so 

 much reduced in size and has become so degenerate in structure 

 that it is quite inconspicuous, and can only be detected by micro- 

 scopical examination and recognized as constituting a distinct 

 generation in the light of our knowledge of lower forms. 



The flowering plant itself is the sporophyte, and it is hetero- 

 sporous, producing microspores and megaspores. The pollen 

 grains are the microspores, while the megaspores are represented 

 by the embryo sacs enclosed within the ovules or unripe 

 seeds. The microspores, like the spores of ferns, are set free 

 from the parent sporophyte, the megaspores, however, are never 

 set free as such, and in neither case does the gametophyte become 

 free from the spore. 



The terms pollen grain and embryo sac were applied to 

 the structures in question long before their true nature as 

 microspores and megaspores was recognized, and they have 

 become so firmly established that it is hardly possible to avoid 

 using them. 



If we examine any typical, fully developed flower, such as is 

 represented diagrammatically in Fig. 53, we shall find that it 

 consists of four whorls or circlets of specially modified leaves. 

 Beginning at the outside we find first the calyx (AY), composed 

 of a number of sepals, which usually, but by no means always, 

 retain the green colour characteristic of leaves and serve mainly for 

 the protection of the inner parts of the flower while in the bud ; 

 then the corolla (K), composed of petals, which may be brightly 

 coloured and serve to attract insects ; then the androecium, 

 composed of stamens (a,/); and lastly, in the centre of the 

 flower, the gynoecium or pistil (n, g, F), composed of carpels. 

 The stamens and carpels are often spoken of as the essential 



