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SCIENCE 



[N. S. Vol. XXXIX. No. 1000 



ing these same facts for plants. Thus 

 Strasburger in 1888 showed that the num- 

 ber of chromosomes characteristic of the 

 egg and of the male nucleus in a number of 

 angiosperms is the same, and is fixed by a 

 reduction, occurring in the mother cells of 

 the pollen and of the embryo-sac. Guig- 

 nard also (1889 and '91) demonstrated 

 these phenomena in Lilium and in the 

 pollen mother cells of Ceratozamia, noting 

 the eight double chromosomes in the latter 

 and other peculiarities of the first mitosis. 

 Overton (1893) counted the same number 

 of chromosomes in the female prothallus of 

 Ceratozamia; while Farmer (1894) found 

 four chromosomes in the thallus and sexual 

 reproductive cells of Pallavicinia and eight 

 in the seta and capsule. 



Later in the same year Strasburger, in 

 a masterly address before the British 

 Association, completed the proof of Over- 

 ton's suggestion (1893) that reduction in 

 the mosses and ferns also takes place, as 

 Overton puts it, ' ' in the mother-cells of the 

 spores; that is, at the point of alternation 

 of the generations. ' ' Strasburger, by com- 

 paring his counts of chromosomes in the 

 dividing spore mother cells of Osmunda, 

 with the number seen by Humphrey (fig- 

 ures published in 1895) in the tapetal cells, 

 found the latter number about double. It 

 is interesting to note also that the Osmunda 

 slides used in this work were among the 

 first paraffin sections used by Strasburger. 



From this correspondence of the liverwort 

 and fern mentioned with the seed plants in 

 which reduction had been seen, Strasburger 

 went on to predict the universal occurrence 

 of this phenomenon of reduction in all 

 plants that reproduce sexually. Concern- 

 ing the phylogenetie origin of the reduc- 

 tion pi-ocess Strasburger held that &l\ 

 plants (and animals) were primitively non- 

 sexual and had a constant number of 

 chromosomes. With the development of 



sexual reproduction the initiation of the 

 process of chromosome reduction avoided 

 the evident disadvantage of repeated 

 doubling of the chromosome number at 

 each sexual fusion. This return from the 

 double number formed in the zygote to 

 the primitive, ancestral number of chromo- 

 somes, he believed might occur at any 

 point in the life cycle before the next 

 fertilization. Strasburger then went on 

 to emphasize the advantage of the sex- 

 ual mode of reproduction, when once ac- 

 quired, in allowing new combinations of 

 parental strains in the offspring, and the 

 disadvantage it had of producing so small 

 a number of offspring. It is to meet this 

 disadvantage, he suggested, in agreement 

 with Bower, that the zygote of forms like 

 Coleochcete, mosses, ferns and seed plants 

 took over the function of multiplying the 

 progeny by a sort of polyembryony, the 

 formation of spores. The spore-bearing 

 generation later in the evolutionary his- 

 tory became ultimately independent of the 

 gametophyte, and at a still later period it 

 not only produced two kinds of spores, but 

 also assumed the care and nutrition of the 

 reduced female plant, arising from the 

 larger of the two kinds of spores. Thus, 

 in Strasburger 's view, the primitive non- 

 sexual generation is now reisresented in the 

 archegoniates by only the sexual phase 

 which has gradually lost its power of asex- 

 ual multiplication, while the sporophyte is 

 a third, a new, generation arisen by spe- 

 cialization of the zygote. There is in the cor- 

 mophytes then an antithetic alternation of 

 the two most recently evolved phases of the 

 life cycle, while the only clear trace of the 

 primitive non-sexual phase is found in the 

 halved number of chromosomes, which is 

 reverted to by a process of chromosome re- 

 duction, at some point in each life cycle. 



In the two decades since this famous 

 pronouncement of Strasburger 's was made 



