CHAPTER XVI 

 CONIFEROPHYTES— CONIFERALES— Co»^mMe(/ 



THE EMBRYO 



In dealing with fertilization we have already begun a description 

 of the embryogeny, for we have described the fertilized egg and some 

 features of its first mitosis. The fertilized egg is the first cell of the 

 2X generation. Although some of the early cells of the 2x generation 

 take no part in the formation of root, shoot, cotyledons or leaves, the 

 entire 2x generation, up to the ripe seed, should be included in any 

 account of the embryo. 



The procmhryo. — In all the living pteridophytes, the first division 

 of the fertilized egg is followed by the formation of a cell wall. There 

 is no free nuclear stage for, throughout the entire group, the egg is 

 small and easily segmented. As the egg increased in size, probably 

 in some extinct heterosporous ancestors of the gymnosperms, a free 

 nuclear stage appeared, and became more prolonged as the egg con- 

 tinued to increase in size. The climax was reached in living cycads, 

 where Dioon shows more than a thousand free nuclei before wall for- 

 mation begins. In the higher cycads, like Zamia, the number of free 

 nuclei becomes as low as 256. In the living Coniferophytes, Ginkgo, 

 with 256 free nuclei, has the highest number. In the conifers the eggs 

 are comparatively small, and the number of free nuclei correspond- 

 ingly low. In only one species in the entire conifer line has a wall 

 been reported at the first division of the fertilized egg. In the im- 

 mense Sequoia sent pervir ens, which has an egg only 100 microns in 

 diameter (almost as small as some of those in living pteridophytes), 

 there is no free nuclear stage. A strong cell plate appears at the 

 telophase of the first division of the fertilized egg, and a distinct wall 

 is formed before the second division occurs (fig. 331). 



In the rest of the conifers, as far as they have been studied, the 

 embryogeny begins with a free nuclear period. 



Finns will serve as a thoroughly investigated example. In the free 



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