416 BOTANY. 



aented by many species during the Tertiary. It is interesting to note 

 tiiat the present small and restricted genus Sequoia was during Cre- 

 taceous and Tertiary times large and widely distributed throughout the 

 northern hemisphere. In this country two Cretaceous species are re- 

 corded from Nebraska and Kansas, and eight species from the Tertiary 

 of Colorado, Utah, Montana, and the region westward. 



The Cycads originated in the Carboniferous, and increased in num- 

 bprs to the Jurassic, in which twenty or more genera were richly repre- 

 sented in species. A Cretaceous species of PUrvphyllum from Nebraska, 

 and a tertiary Zamiostrdbus from Colorado have been described. 



Two species of Ephedra from the Tertiary of Europe are the only 

 known fossil Gnetaceae. 



§ III. Class Angiospbrm^e. 



522. — This class includes the great mass of the so-called 

 flowering plants. The principal characters which set these 

 oif from the preceding small class of the Phanerogams 

 (Gymnospermse), are (1) the development of an ovary, and 

 (3) the aggregation of the reproductive organs into a defi- 

 nite and distinct flower. 



523. — The plants of this class have, in most cases, more 

 or less elongated stems ; these are solid at first, and in the 

 great majority of cases they remain so. They usually bear 

 ample leaves with a parallel (in the Monocotyledons), or 

 netted venation (in the Dicotyledons). The disposition of 

 the fibro-vascular bundles in the stem is either like that in 

 the Gymnosperms (in most Dicotyledons), or they run 

 through the fundamental tissues parallel to, but independent 

 of, one another (in most Monocotyledons). In the former 

 case, the stems of the perennial species increase in diameter, 

 m the same way that they do in Gymnosperms, and there is 

 here also a well-marked division into pith, wood and bark ; 

 in the latter case there is usually no increase in the diameter 

 of the stem after it has elongated, and in tne few cases of 

 considerable increase it takes place by methods very dijBEerent 

 from that described in the preceding class. 



Most Angiosperms are terrestrial and chlorophyll-bearing 

 plants ; there are, however, many aquatic and aerial species, 

 and a considerable number of parasites. They range, also, 

 in size ^nd duration, from minute annuals, a millimetre in 



