The Pteridophytes 487 



a fiat thallus, or a protonema and leafy branched gametophyte. 

 The gametophyte in turn produces oospores which germinate in 

 situ, and a simple parasitic or partially parasitic sporophyte, 

 consisting of a sporangium, foot, and stalk, ensues. Its life ter- 

 minates with the production of spores. 



Among the ferns the asexually formed spores germinate on 

 the soil and produce a prothallus. This is the gametophyte, 

 and it in turn produces an oospore, from which the large, leafy 

 fern plant develops. Among all plant groups, beginning with the 

 Bryophytes, a corresponding alternation of generations may be 

 discerned. 



Chromosome numbers. In Chapter XXXI attention was 

 called to the importance of chromosomes as carriers of heritable 

 qualities. The statement was made that the number of chromo- 

 somes is usually definite, and that at one step (formation of 

 pollen and embryo sac) in the life cycle of complex plants there 

 is a reduction division. Following the reduction division the 

 cells have just one half the number of chromosomes. In the 

 life cycle the last of the cells, with the reduced number, are 

 the sperm and egg. When they unite, forming a zygote or 

 oospore, the number of chromosomes is restored. 



The reduction division occurs in the mother cells that produce 

 the spores. The spores, the cells of the subsequent gametophyte,^ 

 and the sperms and eggs have the reduced number of chromosomes 

 (usually written n number of chromosomes). The oospore and 

 the sporophyte generation up to and including the spore mother 

 cells have the 2 n number of chromosomes.^ 



^ It should be mentioned in this connection that the form of neither the 

 gametophyte nor the sporophyte is determined by the number of chromosomes. 

 These alternate generations are sometimes produced vegetatively in both the 

 mosses and ferns. A gametophyte developed vegetatively from a sporophyte 

 has 2 n chromosomes ; likewise a sporopbyte developed vegetatively from a 

 gametophyte has n chromosomes. 



2 In the brown alga Dictyota the chromosome numbers are i6 and 32; 

 in the red alga Polysiphonia, 20 and 40; in the liverwort Pellia, 8 and 16; 

 in Anthoceros, 4 and 8; in the moss Bryum, 10 and 20. 



