THE HIGHER CRYPTOGAMIA. 341 



the central cell of the archegonium, an open passage is 

 formed leading to the latter cell (PL -XL VI, fig. 5). Before 

 the formation of this passage a free spherical daughter-cell 

 is produced in the central cell, almost filling the cavity of 

 the latter (PL XL VI, figs. 4—6). It is the primary cell of 

 the new generation — the germinal vesicle — capable, after 

 impregnation by the spermatozoa produced in the small 

 spores, of forming a new frond and spore-bearing plant of 

 Isoetes. - . ... - 



All the ripe macrospores of Isoetes lacustris form pro- 

 thallia and produce archegonia. The further development 

 which results in the formation of the embryos of a leaf- 

 bearing plant is attained only by those macrospores which 

 come in communication with microspores. ... This is analo- 

 gous to what occurs in Selaginella and the Miizocarpese. 

 Prothallia when kept quite apart from microspores live for 

 a long time ; .according to my experiments from the begin- 

 ning of September to the middle of March. Some of these 

 even then brought forth new archegonia of the normal form 

 apparently fitted for impregnation. 



The small spores of Isoetes lacustris when ripe have the 

 form of the quadrant of a sphere ; in rare instances they are 

 tetrahedral. . The sharp edges and angles of the spore are 

 formed of exine, those of the inner spore-membrane are 

 bluntly rounded off. At both the upper and lower ends of 

 the spore the exosporium forms a wart-like tip (PL XLV, 

 figs. 8, 9) ; all along the edges of contact of the spore with 

 the three sister-spores it forms a prominent fold (PL XLVI, 

 fig. 7). The outer surface of the exine is very finely granu- 

 lated, almost smooth. Its colour is a light yellowish grey. 



The fully-developed small spore contains a finely granu- 

 lar protoplasm mixed with many small oil-drops. When 

 viewed with transmitted light the mass of differently refrac- 

 tive fluids appears almost opaque. A sharp-outlined sphe- 

 rical nucleus with transparent fluid contents floats in the 

 middle point of the spore (PL XLVI, fig. 7). 



About four weeks after the microspores have become free 

 by the decay of the wall of the sporangia, the primordial utricle 

 of the cell divides into from two to four portions which be- 

 come individualised into daughter-cells (PL XLVI, fig. 8). 



