284 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



The microspore of both groups, like those of the heterosporous Ferns, produce an 

 antheridium and a unicellular rudimentary prothallium. 



The sexual generation (oophore, oophyte]. The microspore of Isoetes lacustris lies 

 dormant during the winter and then divides into a very small sterile cell, and a large 

 cell which encloses the whole of the rest of the cell-contents (Fig. 229 A-C); the 

 smaller cell, invested with a firm wall of cellulose (z>), undergoes no further changes of 

 importance ; the larger cell, the mother- cell of the antheridium, on the contrary divides 

 into four naked primordial cells ; each of the two ventral cells of the four produces 

 two, together therefore four mother-cells of spermatozoids, while the other two cells 

 are displaced and absorbed. In the Selaginelleae also a small sterile cell is first 



FIG. 229. Germination of microspores of Isoetes lacustris, A and C microspores seen from the right side. B and 

 D from the ventral side. A and B show the formation of the antheridium ; SS ' ts dorsal, |3 its ventral cells. C and D 

 show the formation of the spermatozoids, S and ft have disappeared ; -v is everywhere the vegetative cell, the prothal- 

 lium of Millardet. The development of the spermatozoids is indicated by the letters a toy: A D and o ofmagn. 

 580 times, e and/" 700 times. After Millardet. 



separated off by a firm wall some time before the spores fall out of the sporangium, 

 and the other and larger cell, the mother-cell of the antheridium, divides into from six 

 to eight primordial cells (Fig. 2^iA-D). According to Millardet only two inner cells 

 produce the mother-cells of the spermatozoids, and these multiply and displace the 

 others and fill the cavity of the spore ; on the other hand, Pfefifer finds in Selaginella 

 Martensii and S. cauhscens that all the primordial cells first formed in the antheridium 

 divide again and at length give rise to spermatozoids 1 . The spermatozoids in Isoetes 



1 [Belajeff gives a somewhat different account of the development of the antheridium and 

 spermatozoids. In Isoetes the mother-cell of the antheridium is divided by two successive anticlinal 

 walls into a basal, apical and middle lateral cell ; this latter then divides by a vertical wall at right 

 angles to the plane of the previous anticlinals. A periclinal wall in each of the daughter-cells so 

 formed produces two central cells, which are therefore surrounded by four peripheral cells. Each 

 central cell then divides by a transverse wall, and the four daughter-cells are spermatocytes and float 

 in the mucilage resulting from the disorganisation of the outer antheridial cells. These spermatozoids 

 have many cilia attached to the delicate anterior end. In Selaginella the mother-cell of the antheri- 

 dium is first divided into halves by a wall nearly parallel with the axis of the microspore. In each 

 half three successive anticlinal walls appear, separating a basal and apical cell from two intermediate 

 ones. In both of the intermediate cells a periclinal wall separates an inner from an outer cell, and thus 

 the whole antheridium consists of four central cells surrounded by eight peripheral cells (S. Krausiana 

 and S. Poulteri) ; in one only, that next the basal cell, of the intermediate cells in each half is a 

 periclinal wall formed, and there are therefore only two central cells (S. cuspidata, S. laetevirens, 



