ALTERNATION OF GENERATIONS 315 



by the formation of spores, and sexually by means of sex 

 cells or gametes. The male cell is the sperm, the female is 

 the egg or ovum. As is well known, in most plants, and 

 always, normally, in the higher land flora, both types of 

 reproduction occur at definite points in a regular life cycle. 

 A sexual stage, or gametophyte, reproduces by means of 

 gametes. The single cell formed by the fusion of egg and 

 sperm, the zygote, develops into an asexual stage, or sporo- 

 phyte. The spores produced by this give rise to the game- 

 tophyte. This cycle is called the alternation of (sexual and 

 asexual) generations. There may be only one type of 

 gametophyte producing both eggs and sperms, as in many 

 homosporous ferns. In the flowering plants there are 

 male and female gametophytes. In these there are also 

 two types of spore, a microspore which produces only male 

 gametophytes, and a niegaspore which produces female 

 gametophytes. Both types of spore may be produced by 

 the same sporophyte or there may again be two types of 

 sporophyte. 



Intimately connected with this alternation of generations 

 in the higher plants is an alternation in the constitution of 

 the nucleus. The nuclei of the sperm and ovum possess a 

 number of chromosomes, definite in any species ; their fusion 

 results in the formation of a zygote nucleus with double 

 that number, and this double set is maintained in all the 

 cells of the sporophyte. In the formation of the spores 

 the penultimate nuclear division is unique in nature, and 

 results in one-half of the chromosomes passing to each 

 of the daughter cells, so that in these, and in each of the two 

 spores to which each may give rise, only the single set of 

 chromosomes is found. This number is retained in all 

 the cells of the gametophyte. The single set is spoken of as 

 the haploid number, the double as the diploid. 



The angiosperm plant is a sporophyte. It produces 

 two types of spore. Numerous microspores, the pollen 

 grains, are formed in each of the four sporangia normally 

 borne by the stamen. The nucellus, the megasporangium, 

 normally brings to maturity only a single megaspore, the 



