X I'lLICIXE.E LEPTOSPORANGIAT.E 335 



• 

 of PolypodiacCcE, does not seem to be universal.^ The cells of 



both sporogenous cells and tapetum have dense granular 



cytoplasm, and large nuclei. Soon after the divisions in the 



sporogenous complex are completed, the walls of the tapetal 



cells become broken down, and their contents dispersed 



through the large central cavity. The sporangium continues 



to enlarge rapidly after this, and the spore mother cells, still 



united, float in a large cavity, which in the living sporangium 



seems to be filled with a structureless mucilaginous fluid, but 



when fixed and stained is seen to contain the unchanged nuclei 



of the tapetum, as well as its cytoplasmic contents. Gradually 



the connection between the sporogenous cells is lost, and the 



isolated cells, each surrounded by a very delicate membrane, 



float in the large central cavity. Here they divide into four 



cells, as usual, and the division may be simultaneous, resulting 



in tetrahedral spores, or successive {Onoclea), in which case 



bilateral spores are formed. Strasburger "-^ states that during 



the division of the spores in Osniufida there is a reduction of 



the chromosomes to one-half their original number, but whether 



this also occurs in the other Ferns must be left undecided at 



present. Stained microtome sections of sporangia during the 



formation of the spores show that the spore mother cells, and 



afterwards the spores themselves, are embedded in a granular 



matter, evidently the product of the disorganised tapetum, and 



that the nuclei of the latter are collected about them, evidently 



intimately associated with the growth of the young spores, and 



in the later stages with the formation of the perinium. The 



latter is rarely smooth, but shows spines, ridges, and folds of 



characteristic form in different species. 



When chlorophyll is present in the ripe spore it only arises 

 at a late period. In Onodea struthiopteris, about the time that 

 the perinium begins to form, numerous small colourless granules 

 appear near the nucleus, and with the ripening of the spore 

 these increase rapidly in size and number, and an examination 

 shows that the increase in number is the result of division. 

 These are young plastids, and as they enlarge chlorophyll is 

 formed in them, and they become very much crowded, so that 

 the green colour of the ripe spore is very pronounced. 



The further history of the sporangium wall is somewhat 

 •complicated. The stomium, as we have seen, arises from a 



^ Goebel (10), p. 218. - Strasburger {12), p. 293. 



