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. 378, (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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