154 IOWA ACADEMY OF SCIENCES. 



breaks in sedimentation, or the leading cycles of mountain- 

 making activity. These are essentially the horizons of uncon- 

 formities, and they are extended laterally across as much 

 territory as they approximately affected. A large number of 

 less important unconformities are known. The whole, when 

 thus arranged, forms an interlocking series of absolute datum 

 planes by which may be paralleled all geological sections. 



The present scheme is based upon our present plan of 

 geological chronology. In the main this is unchanged, though 

 there is, doubtless, a considerable element of error that will 

 have to be eliminated as the more exact determinations of 

 parallelism are made out. The larger divisions or systems 

 may be left very nearly the same as they are now. The minor 

 subdivisions which cannot now be brought into juxtaposition, 

 can readily be placed in the general scale. This appears to be 

 one of the advantages recommending such a scheme. 



A PRELIMINARY LIST OF THE MOSSES OF IOWA. 



BY T. E SAVAGE. 



The mosses together with the Hepatica3 or liverworts con- 

 stitute the group of plants known as the Bryophytes. This 

 group is distinguished from the Thallophytes, by the fact that 

 they present two modes of reproduction, the sexual and the 

 asexual, which occur in regular alternation. This gives rise to 

 what is called alternation of generations. Most bryophytes 

 also exhibit a very fair differentiation as between stem and leaf. 



The spore of the moss, on germinating, produces a many- 

 celled, branching filament containing chlorophyl, the protonema. 

 From the protonema are developed colorless rhizoids, which 

 penetrate the substratum, and buds which produce the stem or 

 leafy axis of the plant. At the apices of the stems, or of the 

 small lateral branches, are borne the sexual organs, the 

 antheridia and archegonia. Mosses may be monoecious, the 

 antheridia and archegonia being produced on the same plant, 

 or dicjecious, the sexual organs being borne on separate plants. 

 The protonema and the leafy stem with the sexual organs 

 make up the sexual generation. 



