SELAGINELLA. 361 



Note (1) the form and size of the two kiiids of spores 

 (diameter of megaspores many times greater than that of 

 the microspores) both kinds are tetrahedral and rounded, 

 with three radiating lines 011 one side and spiny projec- 

 tions on the outer coat, which is very thick in the case of 

 the megaspore ; (2) the coherence of the microspores in 

 fours (tetrads) until quite ripe ; (3) the structure of the 

 sporangium wall, which is three-layered, the outer layer 

 of radially elongated thick-walled cells, middle layer of 

 small flat cells, the inner layer ( tapetum, which persists 

 until spores are ripe) of elongated but thin-walled cells ; 

 (4) the dehiscence line in the sporangium wall, where 

 the outer cells are shorter than elsewhere ; (5) the ligule 

 of each sporophyll, inserted just outside the short thick 

 stalk. 



512. Germination of Spores. Get ripe spores of 

 both kinds by drying a fresh plant on paper, and sow 

 them together on moist soil or tiles. In a few weeks 

 young Selaginella plants will be seen note (1) the 

 hypocotyl, bearing two cotyledons (each with a small 

 ligule) at its end, and between them the young shoot, 

 which soon branches ; (2) the first root, at either side of 

 which the second and third roots arise from the base of 

 the hypocotyl ; (3) the megaspore, to which the young 

 plant is attached by a foot projecting from the base of the 

 hypocotyl into the tissue of the female prothallus which 

 fills the spore the foot and prothallus can be seen by 

 carefully picking off with needles the ruptured spore-coat 

 and treating with potash. 



513. Development of Prothalli and Sexual Organs. 

 A good deal can be made out by crushing the germinated spores 

 under the cover-glass, or treating them with potash, or (in the case 

 of the megaspores) dissecting off the thick outer coat with needles 

 all these methods should be tried. 



(a) In the microspore the first division (which occurs before 

 the spore leaves the sporangium) cuts off a small lens-like male 

 protliallial cell from a much larger cell; the latter forms the 

 antheridium, consisting of a single-layered wall and a central 

 group of antherozoid-mother-cells. 



