412 BRYOPHYTES (MOSS PLANTS) 
gametes, while the sporophyte generation develops from the fer- 
tilized 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 
Fic. 367.— A diagram showing the life history of Marchantia. Above 
the line (a) is the gametophyte generation and below the line is the sporo- 
phyte generation. p, spore; g, spore germinating to form gametophyte; 
g, mature gametophytes; 0, sex organs; t, gametes; f, 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 
