THE POLYPLOID SERIES IN OSMUNDA 



size relations can be demonstrated for the sporophytes by using the spore mother cells 

 or the epidermis from comparable regions of the leaf. 



Increasing cell size is naturally reflected in increased size of particular organs 

 wherever these depend for their structure on a rather precise cellular arrangement. 

 This is particularly true of the reproductive organs, and gigantism continues to be 

 expressed in the antheridia and sporangia long after it has ceased to appear in the 

 total stature of the plant. This has already been seen in Fig. 1 7 for the antheridia, and a 

 comparable set of sporangia is shown in Fig. 18. 



Table i . Comparative measurements of cells and organs in the autopolyploid 



series q/Osmunda 



Reduction of growth rate is less easy to express in precise terms than are structural 

 characters. An ocular demonstration of it is, however, provided by Fig. 13. This shows 

 the relative speed of development of sexually produced offspring from haploid and 

 from diploid prothalli which were fertilized on the same day and grown on together 

 in one pot. The normals have far outstripped the polyploids. Another feature deter- 

 mined by growth rate is the date of shedding of spores in spring. Under identical 

 conditions of culture, dehiscence of sporangia in diploids and triploids occurs almost 

 simultaneously, with the diploids not more than a day or two in advance of the 

 triploids ; the tetraploids, in contrast, are always about a fortnight later. 



For convenience of reference some of these observations are summarized in Table i, 

 and while it is obvious that many more could be made, especially with regard to the 

 comparative study of physiological processes, enough has perhaps been given to 

 provide a background to the cytological behaviour which, from the present point of 

 view, is the centre of interest. 



The chromosomes of Osmunda are fortunately sufficiently large to provide an almost 

 diagrammatic demonstration of all the more important cytological manifestations of 

 autopolyploidy. The most important of these, for reasons which have already been 

 partly explained in Chapter i, is multivalent pairing at meiosis. In a normal diploid, 

 where only two sets of homologous chromosomes are present, the early stages of the 

 reduction process (prophases* of the first meiotic division) consist in the pairing 



* To those unfamiliar with cytological nomenclature it may be helpful to explain that the words 

 prophase, metaphase, anaphase and telophase are the names given to successive stages of all types of cell division 

 whether mitotic or meiotic. Prophases are the early stages, metaphase is the equatorial plate stage when 



MFC 



33 



