1909] LEAVITT—HOMOEOSIS IN PLANTS 43 



throughout the critical stages, no nuclear fusions were found; and as 

 a matter of fact the chromosomes of the sporophyte continue to be 

 of the reduced (or x) number. Here we have a sporophytic form 

 imposed homoeotically upon a gametophytic cell basis. 



The contrary or aposporic case in ferns has been studied in detail 

 by several workers. In Athyrium F ilix-foemina , Bower 24 found 

 the sporangia arrested at various stages, the development being carried 

 on by prothalloid growths which organized typical wedge-shaped 

 cells at one or more points on their margins, while rhizoids were 

 formed at the same time by the outgrowth of individual cells. 

 Druery 25 had already observed sex-organs and even young plants 

 arising from these prothallia. Wollaston 26 found in Polystichum 

 angulare that the tips of pinnules were converted directly into pro- 

 thallia, bearing archegonia and antheridia. In these cases a game- 

 tophytic form is imposed homoeotically upon a sporophytic cell 

 basis (with little doubt ; though the cytology, I believe, has yet to be 

 worked out) . 



Entirely parallel is the production of protonemata from the 

 setae of mosses, as observed by Pringsheim 27 in Hypnum and 

 Bryum, and by Stahl 28 in Ceratodon. Lang 29 has demonstrated the 

 power of Anthoceros capsules to produce thalli aposporically. 



The adventive structures are, of course, gametophytic, yet they 

 arise by the prolification of sporophytic tissues. 



When 



~>f extraneous characters may be partial 



■P 



Goebel3° has recently induced the formation of prothalloid 



growths from cut fronds of very young plants in several species of 



er ns. Some of the aposporous prothallia bore stomata, as well as 



organs (p. 132). Lang 31 found sporangia in various stages of 



Mourn. Linn. Soc. 21:360. jt886. 

 25 Ibid - 2 i:354. 1886. 



As 



reported by Bower, /. c. 362. 



Akad 



B °t. Zeit. 34:689. 1876. 



^£^<* Botany ,5:503.1901. 



S*zb. Bayer. Akad. Wiss. 37:119-138. 1907. 

 hlU Trans « R °y- Soc. B. 190:194. 1898. 



