THE EVOLUTION OF PLANTS 6oi 
sperms from the Devonian on. The gaps in the evidence 
are always enormous." 
Although the Cycadophyta are now a very insignificant 
element in the earth's flora; in the Mesozoic period they 
form about one-third of the recovered vegetation of the 
land. One order, the Bennettitales, then had a cosmo- 
FIG. 422. Cycadeoidea dacotensis. Semi-diagrammatic sketch of a 
flower (bisporangiate cone), cut longitudinally; one sporophyll folded, and 
one (at the right) arbitrarily expanded. At the center is the apical, cone- 
shaped receptacle, invested by a zone of short-stalked ovules and inter- 
seminal scales. The pinnules of the sporophylls bear the compound 
sporangia (Synangia). Exterior to the flower are several hairy bracts. 
About three-fourths natural size. (After Wieland.) 
politan distribution and seemingly was as important as 
the Dicotyledons are now. Over 30 species of the petrified 
stems have been found in the Mesozoic terrains of the 
United States, the Black Hills of South Dakota alone 
yielding a score. The Isle of Portland forms were called 
