262 



THE SPORE-PRODUCING ORGANS 



[CH. 



at large. The numbers range from thousands of spores in each Eusporangiate 

 sporangium down to a single matured spore in the megasporangia of the 

 Hydropterideae. In the great majority of cases they centre round numbers 

 which fall into the series of powers of two: 8, 16, (24), 32, (48), 64, 128, 

 256, etc. This points obviously to a process of repeated bipartition of the 

 cells of the sporogenous group, terminated by the usual tetrad-division. The 

 figures 24 and 48 do not fall into the series ; they are not uncommon, and 

 an example of the latter is seen in Dryopteris Filix-inas. The number 48 

 probably results from the division of the primary sporogenous cell into 2, 4, 

 and 8 ; only four of the resulting cells then undergo the next division, giving 

 12 spore-mother-cells, and consequently 48 spores (see Fig. 16, 2, 5, 6, 7). A 

 similar process stopping one step earlier in the series of divisions would give 

 the number 24. The figures 32, 16, 8 will follow from sinnilar, but com- 

 plete omissions of the successive divisions in the sporogenous group of cells. 

 Numerical results derived from various Ferns, either based on estimates 

 from sections or on actual countings, are given in the subjoined table, which is 

 arranged progressively from the largest numbers to those which are smaller, 

 while at the same time preserving in some degree the affinities of the Ferns 

 cited. The very numerous records quoted will amply suffice for giving a 

 general conspectus of spore-numbers for living Ferns at large. In the last 

 column the name of the observer is given ; where no name is quoted the 

 observation or estimate was made by the author. 



Authority 



Thompson 



Tansley 



