THE EVOLUTION OF PLANTS 609 



more active points {primordia) appear and begin to de- 

 velop. Soon the whole zone is involved in more rapid 

 growth, resulting in a ring or tube, but with the primordia 

 still evident. The cotyledonary zone continues its growth 

 until a tube of considerable length is developed, leaving 

 the apex of the proembryo depressed. At this stage either 

 one of two things may occur. As the cotyledonary zone 

 continues to grow, the two primordia on the rim of the 

 tube may continue to develop equally, forming two coty- 

 ledons; or one of the primordia may cease to grow, result- 

 ing in an embryo of only one cotyledon; in other words, 

 the entire cotyledonary zone may develop under the 

 guidance of only one growing point. One cotyledon is 

 not eliminated, but the whole growth is diverted into one 

 cotyledon. There thus develops what appears to be an 

 "open sheath" and a "terminal" cotyledon. 



In other words, monocotyledony is not the result of the 

 fusion of two cotyledons, nor of the suppression of one; 

 but is simply the continuation of one growing point on the 

 cotyledonary ring, rather than a division of the growth be- 

 tween two growing points. In a similar way, polycoty- 

 ledony is the appearance and continued development of 

 more than two growing points on the cotyledonous ring. 

 The rudimentary second cotyledon of a "monocotyle- 

 donous" grass-embryo (wheat) is shown in Fig. 37 8, (p. 494). 



523. Ancestors of the Gymnosperms. — As far back as 

 Devonian time, preceding the great coal period (Carbon- 

 iferous) , fossils have been found of a plant, Cordaites (of 

 the order Cordaitales), common in that period, and 

 having characters which indicate that it stands in the 

 ancestral line of our modern conifers — that it and the 

 conifers had a common ancestry. 

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