PAST HISTORIES OF PLANT FAMILIES 161 



existence would have been entirely unsuspected but for 

 the work of fossil botany, are yet the clues which have 

 led to a partial solution of the mysteries surrounding the 

 present-day Lycopods and Equisetums, and which help 

 to bridge the chasm between these remote and degenerate 

 families. 



CHAPTER XVII 



PAST HISTORIES OF PLANT FAMILIES 

 X. The Lower Plants 



In the plant world of to-day there are many families 

 including immense numbers of species whose organiza- 

 tion is simpler than that of the groups hitherto con- 

 sidered. Taken all together they form, in fact, a very 

 large proportion of the total number of living species, 

 though the bulk of them are of small size, and many 

 are microscopic. 



These " lower plants " include all the mosses, and 

 the flat green liverworts, the lichens, the toadstools, and 

 all the innumerable moulds and parasites causing plant 

 diseases, the green weeds growing in water, and all the 

 seaweeds, large and small, in the sea, the minute green 

 cells growing in crevices of the bark of trees, and all 

 the similar ones living by millions in water. Truly a 

 host of forms with an endless variety of structures. 



Yet when we turn to the fossil representatives of 

 this formidable multitude, we find but few. Indeed, of 

 the fossil members of all these groups taken together we 

 know less that is of importance and real interest than 

 we do of any single family of those hitherto considered. 

 The reasons for this dearth of fossils of the lower types are 

 not quite apparent, but one which may have some bearing 

 on it is the difficulty of mineralization. It is self-evident 

 that the more delicate and soft -walled any structure is 



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