ISOETES 



201 



rangia borne in aa excavation in the expanded base of the leaf. The ma- 

 crospores 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 relum. The minute triangular appendage at 

 the upper end of the sporangium is called the ligvle. 



The spores are liberated by the decay of the spo- 

 rangia. They form rudimentary prothallia of two 

 kinds. The microspores produce prothallia with an- 

 theridia, while the macrospores produce prothallia with 

 archegonia. Fertilization takes place as in the mosses 

 or liverworts, and the fertilized egg-cell, by continued 

 growth, gives rise again to the isoetes plant. 



ALTERNATION OF GENERATIONS 



In Chapter XXVI, the alternations of generations 

 and the terms gametophtye 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 new plant- 

 form or generation different from the par- 

 ent 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- 

 gonium, which, although it is attached to 

 the thallus, is not a morphological part 

 thereof. The sporogonium produces spores. It is the sporophyte gen- 

 eration 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. 



The fungi and algse are omitted from these remarks. In the former 

 there is nothing analogous to the sporophyte and the gametophyte. 

 In alga? like spirogyra, evidently the whole plant is a gametophyte, 



370. Isoetes showing habit of 

 plant at a; b, base of leaf 

 showing sporangium, velum, 

 and ligule. 



