VARIATION OF CHROMOSOMES 151 



cross between these two, a hybrid called P. kewensis was 

 produced, which Digby has shown has also 18 chromo- 

 somes. It produced only thrum flowers, and was therefore 

 sterile. Five years later, after this plant had been multi- 

 plied by cuttings, one pin flower appeared which was pol- 

 linated by a thrum flower. It gave rise to the fertile race 

 of P. kewensis, that had 36 chromosomes. What connec- 

 tion there may have been between the hybridization and 

 the subsequent doubling, if there is any connection, is by 

 no means clear. It may be noted that in the reciprocal 

 cross between P. verticillata and P. floribunda, a hybrid, 

 P. kewensis, with 36 chromosomes also appeared. 



The most interesting results on tetraploidy are those 

 of Elie and Emile Marchal on certain mosses, for they 

 have been able to produce tetraploid types experimentally. 

 It may be recalled that in mosses there is an alternation 

 of generations. The diploid (2^^^) generation is known 

 as the sporophyte (Fig. 59) that develops out of and 

 remains attached to the other haploid generation, the 

 gametophyte or moss plant (IN). The sporophyte pro- 

 duces a large number of spores, each containing the half 

 number of chromosomes (IN) as a result of reduction that 

 has taken place in their formation, and from each spore 

 a young moss plant develops, beginning as a protonema of 

 loose threads. When the moss plant produces its heads or 

 flowers the sexual organs appear — archegonia ( 9 ) and 

 antheridia { S). Thus the ' ' sexes ' ' are here represented 

 by the haploid generation. 



The egg-cell, contained in the archegonium, is ferti- 

 lized by a sperm-cell, the antherozooid. The fertilized 

 egg-cell i2N) develops in situ into the straight stalk 

 imbedded at its lower end in the tissue of the moss plant, 

 expanding at its upper end into the cup containing the 

 spores. The mother-cells of the spores — like the tissue of 

 the sporophyte itself — contain the 2N number of chromo- 

 somes, which, by two divisions (similar to these already 

 described for the animal cells during reduction), reduces 



