172 Gigantism in Primula sinensis 



in the giant and in the normal varieties, no detailed histological study 

 of the two forms has, as yet, been undertaken. 



The rough histological analysis, which has been made, suffices, 

 however, to prove that in the variety Giant White Queen Star, the 

 gigantism of the individual is the expression of that of its con- 

 stituent cells. The mutant is a giant because its cells are gigantic. 

 Further, the facts that, in the cortex, the number of cell-layers is 

 fewer in the giant than in the normal plant, and that the rate of 

 growth of the plant as a whole is slower in the former than in the 

 latter, suggest that cell gigantism may be due to a reduction in the 

 normal rate of cell-division. 



Examination of Figs. 2 — 5, shows that the cells of the giant are 

 larger than those of the normal form in all three dimensions — radial, 

 tangential, and longitudinal, and that gigantism is not peculiar to the 

 elements of any one tissue-system, but is common to the cells of 

 all — epidermal, cortical, and stellar. 



The extent of the differences in size between the cells of the giant 

 and ordinary White Queen Star is illustrated by the following figures, 

 in which the size of a giant's cell is represented by 100. 



Flower peduncles (see Figs. 2 and 8, and Plate XI). Cortical cells of 

 the layer immediately external to the endodermis : — 



Giant : Normal 



Radial measurement 100 : 48 



Tangential „ 100 : 81 



Longitudinal „ 100 : 57 



V. The Genetical Behaviour of Giant White Queen Star: with 

 Notes on the Genetics of Gigantism in P. sinensis. 



The race, Giant White Queen Star, the origin of which is described 

 in Section III, differs markedly from the parent race with respect to 

 fertility. Whereas plants of normal White Queen Star produce seed 

 freely both when self-fertilized and when crossed with other varieties 

 of P. sinensis, the giant race is relatively sterile. Nevertheless self- 

 fertilization of the original giants (1908, Table II) resulted in the 

 production of enough seed to continue the strain. The partial self- 

 sterility which characterised the first generation of giants has not been 

 maintained, and the descendants of the original Giant White Queen 

 Star now yield when self-fertiUzed a fair amount of seed. 



