IX riLICINE^ LEPTOSPORANGIATAE 343 



cells, still united, lloat in a lar^^e cavity, whicli in the living 

 sporangium seems to be filled with a structureless mucilaginous 

 fluid, but when fixed and stained is seen to contain the un- 

 changed nuclei of the tapetum, as well as its cytoplasmic con- 

 tents. Gradually the connection between the sporogenous cells 

 is lost, and the isolated cells, each surrounded Ijy 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 ((12), 

 p. 239) states that during the division of the spores in Osmimda 

 there is a reduction of the chromosomes to one-half their orig- 

 inal number, but in a later paper (14) he reports that although 

 there is a reduction in the number of chromosomes, the ratio of 

 tw^elve to twenty-four, which was first given, is not absolutely 

 constant. 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 w^ith 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 Onoclea striithiopteris, about the time that 

 the perinium begins to form, numerous small colourless gran- 

 ules appear near the nucleus, and \vith the ripening of the spore 

 these increase rapidly in size and number, and an examination 

 show^s 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 

 special cell of the last-formed lateral segment. The segment 

 on the opposite side (next older but one) shows a quite similar 

 arrangement of cells, and, according to IMiiller, the cell corre- 

 sponding to the stomium by two transverse walls forms the 

 first segment of the annulus. The cells immediately below also 

 divide similarly, and give rise to a second section. The rest of 



