226 MORPHOL OG Y. 



tion 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 (sporo- 

 phyte). Here the single-cell stage is again reached in the spore, 

 which now develops the thallus. 



482. Riccia compared with coleochsete, cedogonium, 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 coleochaete 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 coleochsete again. In coleochaete then we have two phases of the 

 plant, each having its origin in a one-celled stage. Then if we go back 

 to cedogonium, we remember that the fertilized egg, before it developed 

 into the cedogonium plant again (which is the gametophyte), at first divides 

 mtofour cells which become zoospores. These then develop the cedogonium 

 plant. 



Note. Too much importance should not be attached to this seeming ho- 

 rology of the sporophyte of cedogonium, coleochsete, and riccia, for the nu. 

 clear phenomena in the formation of the zoospores of cedogonium and coleo. 

 chaete 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 cer- 

 tain features of the structure of the liverworts. 



The plants are of two kinds, male and female. The two dif- 

 ferent organs, then, are developed on different plants. In 

 appearance, however, before the beginning of the structures 

 which bear the sexual organs they are practically the same. The 

 thallus is flattened like nearly all of the thalloid forms, and 

 branches in a forked manner. The color is dark green, and 

 through the middle line of the thallus the texture is different 

 from that of the margins, so that it 'possesses what we term a 



