THE REDUCTION OF THE CHROMOSOMES 



225 



In VASCULAR PLANTS reduction in all normal life cycles in both homo- 

 sporous and heterosporous forms occurs uniformly in the divisions differ- 

 entiating the spore tetrads (Fig. 77). The sporocytes, particularly the 

 microsporocytes ("pollen mother-cells"), of the higher plants have long 

 been favorite objects for the study of reduction. Since the gametophyte 

 generation in the higher plants is so abbreviated, reduction closely pre- 

 cedes fertilization in these forms. In the ordinary angiosperm embryo 

 sac in which the eight nuclei are derived from a single megaspore of the 

 tetrad, the egg nucleus is removed from the product of reduction (mega- 

 spore nucleus) by only three mitoses. In some cases, of which Lilium 

 is the best known example, walls fail to form be- 

 tween the four megaspore nuclei (Fig. 80, B], 

 leaving them in a common cavity (embryo sac) 

 where they undergo but one further division to 

 produce the eight nuclei of the female gameto- 

 phyte. The egg here is consequently removed 

 from the product of reduction by a single 

 mitosis. In one known case, Plumbagella 

 (Dahlgren 1915), the four reduced nuclei, 

 formed as in Lilium, divide no further, one of 

 them functioning directly as the egg nucleus. 

 Here, therefore, the condition characteristic of 

 animals has been reached : the gamete nucleus is 

 itself the direct product of reduction, and the 

 haploid generation usually produced by the 

 spore is eliminated. The male gametophyte 

 also has undergone much abbreviation in higher 



plants, but the male nucleus is still removed from the reduction product 

 (microspore nucleus) by two mitoses. In no known case does the micro- 

 spore nucleus function directly as a gamete nucleus. 



The term gonotokont was introduced by Lotsy (1904) to designate 

 any cell, whatever its origin or position in the life cycle, in which the 

 reduction process is initiated. In animals the gonotokonts are therefore 

 the primary spermatocyte and the primary oocyte. In most green algs& 

 the gonotokont is the zygote; in the red algae it is usually the tetrasporo- 

 cyte; in the ascomycetes it is the ascus; in the basidiomycetes it is the 

 basidium ; and in the bryophytes and vascular plants it is the sporocyte 

 the microsporocyte and megasporocyte in the case of heterosporous forms. 



The Meaning of Reduction. In order that the true meaning of reduc- 

 tion may be appreciated it will be necessary to indicate the main points 

 of a theory first suggested by Roux (1883) and later developed particu- 

 larly by Weismann (1887, 1891, 1892). It had been believed by the earlier 

 workers that reduction was merely a process whose function was "to 

 prevent a summation through fertilization of the nuclear mass and of 



15 



A 



FIG. 80. Megaspore 



tetrads in Angiosperms. 



A, tetrad of walled cells 



in Physostegia virginiana; 



formation of two upper 



ones just being completed. 



X462. (After Sharp, 1911.) 



B, tetrad of megaspore 



nuclei in Lilium canadense. 



