THE POLYPLOID SERIES IN OSMUNDA 



Some idea of the gross morphology of the various members of both generations 

 may be obtained from Figs. 14 and 15. Each member of the series, even a sterile pro- 

 thallus, is a complete individual provided with all the appropriate vegetative and 

 reproductive organs, though perhaps the female sex organs may not always be struc- 

 turally perfect. Differences in size, shape, growth rate and no doubt other physiological 

 processes nevertheless accompany polyploidy. 



The first increase in chromosome number produces gigantism; this is conspicuously 

 the case for the triploid sporophytes and the diploid gametophytes, both of which are 

 larger than the corresponding normal individuals. With further increase of chromo- 

 some number size is again reduced. The triploid prothalli are almost as large as 



Fig. 17. Comparative sizes of antheridia and spermatozoids in tetraploid (a) 

 and haploid {b) prothalli of Osmunda. x 1000. 



the diploid though not quite so regular in appearance, but the tetraploids are smaller 

 even than the haploids, and the tetraploid sporophytes are depauperate in a similar 

 degree. 



The high-numbered forms are not only depauperate but show structural irregularities. 

 Thus the tetraploid prothalli display a highly characteristic fimbriation of the margin, 

 which is foreshadowed to a less extent in the triploids. The equivalent effect in the 

 tetraploid sporophytes appears as an irregular lobing of the leaf. 



These irregularities are likely, in part at least, to be the expression of increased cell 

 size coupled with reduced growth rate. Cell size increases continuously, throughout 

 the polyploid series. Fig. 16 shows the series of rhizoid diameters for the four types of 

 prothalli and Fig. 1 7 gives the extreme spermatozoid sizes. The large spermatozoids of 

 the tetraploid gametophytes have been seen to swim though with a slow and clumsy 

 motion; the reason for the sterility of these prothalli is therefore not obvious. Similar 



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