L YCOPODINEAE.—LIGULA TAE. 



285 



are long and slender, becoming attenuated and splitting into a pencil of long slender 

 cilia at both ends ; those of Selaginella are shorter, thick at the posterior extremity but 

 tapering gradually at the anterior, and there divided into two long delicate cilia ; the 

 spermatozoids when fully developed are coiled up into 

 a longer or shorter spiral. The mode of formation in 

 the mother-cells is the same in both genera and agrees 

 with that of the Ferns. The spiral body of the sperma- 

 tozoid is obtained by the splitdng up of the protoplasm 

 of the thickened periphery of the nucleus of the mother- 

 cell, proceeding from the anterior to the posterior ex- 

 tremity ; the spermatozoid when formed lies coiled round 

 a central vacuole, which invested with a delicate mem- 

 brane often remains hanging to the posterior extremity 

 of the spermatozoid after it has escaped from the mother- 

 cell, and is carried about with it. The spermatozoids of 

 Isoetes continue only about five minutes in movement, 

 those of Selaginella from half to three quarters of an 

 hour. About three weeks elapse from the beginning of 

 germination to the complete development of the sperma- 

 tozoids in Isoetes, and the same time is required in 

 Selaginella, reckoning from the dispersion of the spores. 



The macrospores produce the female prothallium, 

 which is an endogenous formation in a sdll higher 

 degree than it is in the heterosporous Ferns ; in this 

 respect and in the mode of its development it shows a 

 still greater resemblance to the prothallium in the macrospores (the embryo-sac) of 

 the Gymnosperms and even of the Angiosperms. A few weeks after the macrospores 

 oi Isoetes 2a& set free from the decaying macrosporangium their interior begins to fill 

 with cell-tissue, the cells of which are at first all naked and without a cell-wall ; it 

 is not till the endosporium is quite filled with them that they are seen to be bounded 

 by firm walls (Fig. 230). Meanwhile the endosporium becomes thicker and is dif- 

 ferentiated into layers, and assumes a finely granular appearance ; these phenomena, 

 as Hofmeister has pointed out, are all seen in the embryo-sac of the Coniferae. 

 Then through the swelling of the spherical prothallium the three coherent edges of 

 the exosporium separate longitudinally and produce a three-rayed aperture, where 

 the .prothallium is now covered only by the endosporium ; this too ' peels off' and 

 softens, and finally allows the underlying portion of the prothallium to become ex- 

 posed to the air. On its apex appears the first archegonium ; if its oosphere is not 

 fertilised several others may be formed by its side. While the macrospores of the 

 Selaginelleae are still lying in the sporangium, their apical region is occupied by a 

 small-celled meniscus-shaped tissue, formed probably during the maturing of the 

 spores by the breaking up of a quantity of protoplasm collected there. It is this 

 tissue which subsequently produces the archegonium, and is therefore the true 



S.fukmta, S. stolonifera, S. Martensii. S. viticidosa, S. inaequalifolia, S. canlesceiis). Many divisions 

 . in the central cells produce a complex of spermatocytes, which by the breaking down of the inner 

 walls of the peripheral cells floats in a quantity of mucilage within the outer antheridial wall.] 



Fig. 230. Isoetes lacustris. ^ niacrc 

 spore two weeks after beingf placed ii 

 glycerine and become transparent. B lor 

 (,'itudinal section of the prothallium aftc 

 four weeks in glycerine ; a archegoniun 

 A niagn. 60, B 40 times. 



