412 



BRYOPHYTES (MOSS PLANXS) 



gametes, while the sporophyte generation develops from the fer- 

 tiHzed egg and produces the spores. It is obvious that this is 

 alternation of generations. Some of the higher Red Algae, as 

 illustrated by Polysiphonia, have an alternation of generations 



Fig. 367. — A diagram showing the Ufe history of Marchantia. Above 

 the line (a) is the gametophyte generation and below the line is the sporo- 

 phyte generation, p, spore; q, spore germinating to form gametoph3fte; 

 g, mature gametophytes; o, sex organs; t, gametes; /, fertilized egg or first 

 cell of the sporophyte; m, fertilized egg dividing; n, mature sporophyte 

 ready to shed spores which are the first cells of new gametophytes. 



in their life cycle, but in Bryophytes this feature is- so well estab- 

 lished that it occurs everywhere in the group and is so evident 

 that it was in the Bryophytes that the alternation of generations 

 was first observed. Alternation of generations is also an estab- 

 lished feature of Pteridophytes and Spermatophytes or all plants 

 above the Bryophytes. 



One of the interesting features in connection with the transition 

 from the gametophyte generation to the sporophyte generation 

 is a peculiar kind of cell division known as the reduction division. 

 It will be recalled that in preparation for cell division the chro- 

 matin in the nuclei of cells forms into a definite number of chro- 

 mosomes, the number depending upon the kind of plant. Now 

 reckoning nuclear content in terms of chromosomes, it is obvious 

 that since fertilization is a fusion of the nuclear contents of a 

 sperm and an egg, the number of chromosomes in the nucleus of 



