Alternation of Generations in Ectocarpus siliculosus

Abstract

1. An alternation of generations occurs in the life cycle of Ectocarpus siliculosus growing in the region of Woods Hole. 2. The diploid asexual plants bear unilocular and plurilocular sporangia on separate individuals or simultaneously on the same individual. In the plurilocular sporangium are formed diploid zoo-spores which germinate directly into other asexual plants. 3. A reduction division occurs in the unilocular sporangium and the haploid zoids formed in this organ germinate into haploid sexual plants. 4. The sexual plants are dioecious and produce physiologically anisogamous gametes in plurilocular sporangia. The zygotes develop into diploid asexual plants. 5. The sexual generation propagates itself by the parthenogenetic development of about 5 per cent of the unfused gametes of either sex. 6. Haploid plants are inferior to diploid plants in stature, size of cells, and size of plurilocular sporangia. 7. Plants were grown in suspended drops from spore to spore. Some cultures lived for more than six months. 8. Sexual plants were found only at Penikese Island where they occur on Chordaria. Asexual plants are widely distributed in the region of Woods Hole and occur on Chorda, Chordaria, and Spartina. 9. It is suggested that the sexual plants are obligate epiphytes on Chordaria and that their absence at Woods Hole proper is owing to the absence of this host in this locality.