XXIII. THE EMBRYOGENY OF GINKGO. 



HAROLD L. LYON. 



INTRODUCTION. 



The account published by Coulter and Chamberlain in 1901 

 is the most recent work on the embryogeny of Ginkgo and may 

 be taken as a summation of our knowledge on the subject up to 

 the present time. They wrote as follows, " Although the 

 embryo of Ginkgo is exceptional among Gymnosperms, and of 

 great interest, the details of its development are not sufficiently 

 known. We have been able to secure almost a complete series 

 showing the general outlines of the development, which merely 

 confirms the facts already published. Germination of the 

 oospore begins, as is usual, among Gymnosperms, with repeated 

 nuclear divisions. These nuclei, however, instead of organiz- 

 ing a parietal tissue as in the Cycads, or a basa 1 group as in the 

 Conifers, proceed to fill the whole cavity of the enlarging 

 oospore with free nuclei, which is followed by the organization 

 of a compact tissue. In a certain sense this structure would 

 seem to represent the proembryo of Cycads, but it really repre- 

 sents the whole product of the oospore, in which proembryo, 

 suspensor, and embryo proper are not differentiated. The com- 

 plete filling of the spore with tissue, and the lack of early dif- 

 ferentiation into the great embryonic regions, would suggest 

 a habit more primitive than in either Cycads or Conifers. At 

 the same time, it may be merely a derived character. In any 

 event, the tissue near the base of the spore, which in the other 

 groups develops both suspensor and embryo, shows far greater 

 vigor than the remaining tissue. In the organization of the 

 embryo the whole mass of tissue is involved, and in the absence of 

 a suspensor the embryo invades the endosperm by direct growth. 



The two cotyledons are differentiated early in October, and 

 are quite unequal in length. The larger one is two lobed, while 

 the shorter one is cleft halfway down, thus early indicating the 

 bilobed character of the leaf. The two cqtyledons are also 

 united at the apex, but the epidermal layers of the two are dis- 



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