84 MITOSIS : THE VARIATION OF THE CHROMOSOMES 



in relation to meiosis and polyploidy what certain of these needs 

 are. The most obvious, however, may be mentioned here. The 

 minimum size of the cell in the life history clearly limits the size 

 of the chromosomes, on the principle that you cannot put a quart 

 into a pint pot. The effect of this is seen in various ways. In 

 polyploids the area of the metaphase plate does not increase in 

 proportion to the cell volume even where this increases in proportion 

 to the chromosome number. Tetraploids do not occur in some 

 genera with the largest chromosomes, such as Lilium and Fritillaria. 

 In Tulipa a pentaploid is found in one species only, that with the 

 smallest chromosomes {T. Clusiana) and tetraploids are not found 

 in those with the largest chromosomes, the garden tulips. Poly- 

 ploidy is commonest in the Dicotyledons where the chromosomes 

 are smallest. In Dianthus the difficulty seems to have been over- 

 come by the polyploid species having smaller chromosomes than 

 their diploid relatives. They are genetically adapted. In con- 

 sidering limitation of cell-size we have to remember the whole life- 

 history of the organism. In plants the most serious restriction 

 occurs in the narrow cambium cells which produce secondary 

 growth. It is therefore not without significance that secondary 

 growth is not found in organisms with the largest chromosomes, the 

 Gymnosperms being at the upper limit in this respect. Apart 

 from sections of the Ranunculaceae, Berberidaceae and Droseraceae, 

 the Dicotyledons have small or medium-sized chromosomes. Those 

 Monocotyledons with secondary growth have small chromosomes or 

 very few large ones. Three evolutionary relationships seem to 

 be explained by these considerations : First, the size of the cells 

 conditions the size of the chromosomes. Secondly, the size of the 

 chromosomes conditions the changes of shape of the cells at 

 particular phases of growth, and so limits the internal habit of 

 growth. Thirdly, the multiplication of the chromosomes by 

 polyploidy is conditioned by the existence of a margin of space in 

 the cell permitting an increase in the bulk of the chromosomes. 



