SPORE-PRODUCTION 23 



still remains at the centre, having ' rown meanwhile, undergoes successive 

 divisions till usually twelve spore-mother-cells are formed (Fig. 6. 2, 6, 7) : 

 these become spherical in form, and are suspended in a fluid which, 

 together with the now disorganised tapetum, fills the enlarged cavity of 

 the sporangium. Each spore-mother-cell then divides twice, so as to form 

 a group of four cells, which constitute a spore-tetrad (Fig. 7), the component 

 cells showing some differences in their arrangement. Finally, as ripe- 

 ness is approached the individual cells of the tetrads separate as the 

 spores, each of which has meanwhile developed a protecting wall : owing to 

 the absorption of the fluid contents of the sporangium the separate spores 

 are dry and dusty, and readily scattered. Since each 

 of the 12 spore-mother-cells may form four spores 

 their number is 4x12 = 48 in each sporangium. 

 Each mature spore consists of a protoplast with 

 nucleus, bounded by a colourless inner wall, and 

 a brown epispore, which extends outwards into 

 irregular projecting folds. 



Meanwhile the wall of the sporangium has 

 become differentiated into the thinner lateral walls 

 of the lens-shaped head, and the annulus, which 

 is a chain of about 16 cells surrounding its margin 



/T ^. , 7X rr.1 ,-, i i Spore-tetrads of Polypodium 



(Fig 6. 40, 40). These constitute a mechanical vu i are. (After Atkinson.) 

 spring, which on the rupture of the thin-walled 



stomium becomes slowly everted as the cells dry in the air, and then 

 recovering with a sudden jerk, throws out the spores to a considerable 

 distance (Fig. 8), each individual spore being separate from its neighbours. 

 If a Fern leaf on which the sori are fully matured be laid with its lower 

 surface downwards upon a sheet of paper, and left in dry air for some hours, 

 or if the drying be accelerated by heat, a fine brown dust, consisting of 

 the mature spores, will be deposited on the paper, and they are shed in 

 such vast numbers as to give a natural print of the outline of the leaf. 

 A rough estimate may be made of the numerical output of spores from 

 a large plant of the Shield Fern, as follows. In each sporangium 48 

 spores may be formed: a sorus will consist of fully 100 sporangia, usually 

 more : 20 is a moderate estimate of the sori on an average pinna : there 

 may be fully 50 fertile pinnae on one well-developed leaf, and a strong 

 plant would bear 10 fertile leaves. 48x100x20x50x10 = 48,000,000. 

 The output of spores of a strong plant in the single season will thus, on 

 a moderate estimate, approach the enormous number of fifty millions. 



As we shall see, each of those spores is capable of acting as the starting- 

 point of a new individual, and yet Male Ferns are not increasing perceptibly 

 in number : the fact is that in open Nature the vast majority of these 

 potential germs do not survive the vicissitudes of early life. It is evident, 

 however, that the maintenance of the race is very fully provided for, while 

 there is an ample margin for the effect of selection of those fittest to survive. 



