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TEXTBOOK OF BOTANY 



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Fig. 363. Restorations of Carboniferous plants: A, Lepidodendrou, a tree re- 

 lated to the club mosses; B, Cordaites, an ancient gymnosperm. Copyright by 

 Field Museum of Natural History. 



triple-fusion nuclei, and endo.speims is comparatively recent. Some of 

 the features found in flowering plants, however, had been evolving 

 since pre-Cambrian times. The reader may find it interesting to specu- 

 late about the relative ages of some of these features. How old, for 

 example, are the processes of seed formation, formation of pollen, 

 cambial growth, development of leaves, organization of cells in a multi- 

 cellular individual, sex, mitosis, spore formation, chlorophyll synthesis, 

 vegetative multiplication, and respiration? Have these inherent proc- 

 esses remained unaltered since the time of their first appearance? 



Mesozoic floras. Among the fossfls in Triassic and Jurassic rocks are 

 those of two major groups: the Bennettitales and the primitive cycads. 

 The former are gymnosperms without obvious internodes, and with an 

 apical crown of large compound leaves, in the axils of which are cones 

 of sporophylls having terminal pollen sacs and ovules. The stems and 

 leaves of the cycads were similar to those of the Bennettitales, but the 

 pollen sacs and ovules occurred on leaves arranged in a terminal cone. 



