Riccia glauca; archegonium containing nearly mature sporogonium. _sg_, spore-producing cells surrounded by single layer of sterile cells, the wall of the sporogonium.] =481. A new phase in plant life.=—Thus we have here in the sporogonium of _riccia_ a very interesting phase of plant life, in which the egg, after fertilization, instead of developing directly into the same phase of the plant on which it was formed, grows into a quite new phase, the sole function of which is the development of spores. Since the form of the plant on which the sexual organs are developed is called the _gametophyte_, this new phase in which the spores are developed is termed the _sporophyte_. Now the spores, when they germinate, develop the _gametophyte_, or thallus, again. So we have this very interesting condition of things, the thallus (gametophyte) bears the sexual organs and the unfertilized egg. The fertilized egg, starting as it does from a single-celled stage, develops the sporogonium (sporophyte). Here the single-cell stage is again reached in the spore, which now develops the thallus. =482. Riccia compared with coleochæte, œdogonium, etc.=—We have said that in the sporogonium of riccia we have formed a new phase in plant life. If we recur to our study of coleochæte we may see that there is here possibly a state of things which presages, as we say, this new phase which is so well formed in riccia. We recollect that after the fertilized egg passed the period of rest it formed a small rounded mass of cells, each of which now forms a zoospore. The zoospore in turn develops the normal thallus (gametophyte) of the coleochæte again. In coleochæte then we have two phases of the plant, each having its origin in a one-celled stage. Then if we go back to œdogonium, we remember that the fertilized egg, before it developed into the œdogonium plant again (which is the gametophyte), at first divides into _four_ cells which become zoospores. These then develop the œdogonium plant. Note. Too much importance should not be attached to this seeming homology of the sporophyte of œdogonium, coleochæte, and riccia, for the nuclear phenomena in the formation of the zoospores of œdogonium and coleochæte are not known. They form, however, a very suggestive series. Marchantia. =483.= The marchantia (M. polymorpha) has been chosen for study because it is such a common and easily obtained plant, and also for the reason that with comparative ease all stages of development can be obtained. It illustrates also very well certain features of the structure of the liverworts. [Illustration: Fig. 257. Male plant of marchantia bearing antheridiophores.]