226 MORPHOLOGY. 



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 coleochaete, oedogonium, etc. — We have said 

 tliat in the sporogonium of riccia \vc liavc formeil a new jiha^e in plant life. 

 If we recur to our sluily of coleochaete we may see that there is here possibly 

 a state uf thini^s which presages, as we sa}', this new phase wliicli 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 coleochffite again. In coleochcete then we have two phases of the 

 Iplant, each having its origin in a one-celled stage. Then if ^\e go liack 

 to cedogonium, we rememl>er that the fertilized egg, hefore it developed 

 into the cedogonium plant again (which is the gametophyte), rit lirst divides 

 uAofoit?- cells which become ztjospiores. These then develop the cedogonium 

 plant. 



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

 mology of the spon^phyte of (edogonium, coleochiete. and riccia, fjr the nu- 

 clear phenomena in the formation of the zt^ospc-res of (edc>gonium and colco- 

 chsete 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 ol 

 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 jilants. In 

 appearance, however, before the beginning of the structures 

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

 thallus is flattened like nearl\' all of the thalloitl forms, and 

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

 through the nuddle line of the thallus the iexture is different 

 from that of the margins, so that it [lossesses what we term a 



