194 



STUDIES IN CEYPTOGAMS 



Isoetes bears two kinds of spores, large roughened ones, the 

 macrospores, and small ones or microspores. Both kinds are formed 

 in sporangia borne in an excavation in the expanded base of the leaf. 

 The macrospores are formed on the outer, and the 

 microspores on the inner leaves. A sporangium in 

 the base of a leaf is shown at b. It is partially 

 covered by a thin membrane, the velum. The mi- 

 nutetriangular appendage at the upper end of the 

 sporangium is called the ligule. 



The spores are liberated by the decay of the 

 sporangia. They form rudimentary prothallia of 

 two kinds. The microspores produce prothallia 

 with antheridia, while the macrospores produce 

 prothallia with archegonia. Fertilization takes 

 place as in the mosses or liverworts, and the fer- 

 tilized egg-cell, by continued growth, g^ves rise 

 again to the isoetes plant. 



ALTERNATION OF GENERATIONS 



In Chapter XXIV the alternation of 

 generations and the terms gametophyte and 

 sporophyte were explained. In many of 

 the plants just studied, this alternation 

 is more clearly and beautifully marked 

 than in any other groups of plants. In 

 each generation, the reproductive body 

 (egg or spore) gives rise to a mew plant- 

 form or generation different from the 

 parent generation. In the liverworts the 

 thallus produces the egg. The fertilized egg-cell is the beginning of 

 a new plant, but this new plant is not like the thallus which produced 

 the egg, nor does it lead an independent existence. It is the sporo- 

 goniuin, which, although it is attached to the thallus, is not a mor- 

 phological part thereof. The sporogonium produces spores. It is the 

 sporophyte generation of the plant, and not until the spores germinate 

 is the thallus again produced. The same is true in the mosses. The 

 "moss plant" produces the egg-cells. It is the gametophyte. The 

 fertilized egg-cell develops into the sporophyte the spore-case and 

 its stem. We can pull the stem of the capsule out of the moss plant 

 and thus separate the sporophyte from the gametophyte. 



343 



Isoetes showing habit of 

 plant at a; b, base of leaf 

 showing sporangium, vel- 

 um, and ligule. 



