EVOLUTIONARY TENDENCIES AMONG GYMNOSPERMS 427 



condition the free nuclei are so numerous that wall-formation results 

 in a tissue that fills the egg. The tendency is to reduce the number 

 of free nuclear divisions, resulting in a reduction of the amount of 

 proembryonic tissue, so that more and more of the general cytoplasm 

 of the egg is left free from the tissue. The proembryo of Ginkgo 

 has retained a very primitive character, and illustrations of early 

 stages in the reduction of the proembryo may be observed among 

 the cycads. In Zamia, for example, which illustrates the extreme 

 amount of reduction among cycads, the proembryo is a relatively 

 small amount of tissue at the base of the egg. Among the Conif- 

 erales this reduction has been carried forward to a much greater 

 extent, sixteen free nuclei appearing sometimes, eight free nuclei 

 being the prevailing number, and four free nuclei being occasionally 

 attained. This results in a proembryo consisting of a small and 

 definite number of cells, with distinctly distributed functions. Now 

 and then among gymnosperms (as in Sequoia) there is no free nuclear 

 division, but these are interesting rather than significant exceptions. 

 Even among the Gnetales it is found that a certain amount of free 

 nuclear division precedes wall-formation. That this reduction of 

 the free nuclei before wall-formation is significant is shown by the 

 fact that among angiosperms free nuclei have disappeared, and wall- 

 formation accompanies the first division of the fusion nucleus. 



The vascular structure of seedlings, and especially the vascular 

 plate with its cotyledonary connections, has become so important a 

 factor in discussions of phylogeny, that at our request Sister Helen 

 Angela has prepared the following brief comparative statement of 

 the situation among gymnosperms. 



" Since the hypocotyl, cotyledons, and primary root are the first 

 organs formed in the embryo proper, it is reasonable to suppose that 

 they depart less widely from the generalized condition than do those 

 structures which develop later under the influence of the various 

 external factors that tend to create diversity. If in addition we 

 could say that developmental factors are the same within all seeds, and 

 that all embryos develop under similar conditions, we should have 

 strong ground for broad generahzations. But such a statement, of 

 course, would be far from the truth. Conspicuous examples of the 

 factors which must be reckoned with in any such attempt are the 



