860 



SCIENCE 



[N. S. Vol. XLII. No. 1094 



Torreya, for example, as soon as there are 

 walled cells at all, the arehegonium initials 

 become recognizable; and all of the tissue 

 -development (the so-called endosperm) ap- 

 pears after the arehegonia are under way. 



Finally the shift is made into the first 

 ■stage of gametophyte development, that of 

 free nuclear division, so that, of course, 

 there is no arehegonium initial, and no 

 arehegonium, as in Gnetum, the egg being 

 organized in connection with a free nucleus. 

 If one were to continue this progress into 

 Angiosperms, he would find eggs appearing 

 •earlier and earlier in the free nuclear stage 

 ■of the gametophyte, so that the free nuclei 

 are relatively few when eggs are organized ; 

 and recently a form has been found in 

 "which the megaspore nucleus organizes an 

 ^egg directly, so that this backward move- 

 ment in ontogeny has reached its limit, at 

 least in one extreme ease. 



Such a series of progressive changes in 

 gymnosperms, and there are several others 

 equally distinct, furnishes us perhaps our 

 most impressive illustration of what Naegeli 

 called "progressive evolution," which we 

 have come to call orthogenesis. Here is a 

 steady progress in a given direction through 

 an immeasurable lapse of time, during 

 which, presumably, the plants have been 

 exposed to every conceivable change of con- 

 ditions. 



Recent experimental work upon sexual- 

 ity in plants, however, may suggest an ex- 

 planation for this phenomenon among 

 gymnosperms. It is now known that the ap- 

 pearance of gametes (the sexual cells) is in 

 response to certain conditions affecting 

 metabolism. When the gamete is associated 

 with a sex organ, as the arehegonium, the 

 conditions for gamete formation are the 

 conditions for arehegonium formation. In 

 other words, the essential response is the 

 gamete ; a sex organ may or may not be in- 

 volved. Speaking in very general terms, 



the conditions that favor gamete forma- 

 tion are associated with minimum vegeta- 

 tive activity. These conditions may affect 

 the plant as a whole, in such forms as algae, 

 or only certain protoplasts, and in them the 

 sex response occurs. It follows in the case 

 of our gymnosperms that any change of 

 conditions shortening the period of vegeta- 

 tive activity, would thereby hasten the ap- 

 pearance of eggs in the ontogeny of the 

 gametophyte. This is exactly the result 

 that would follow the differentiation of the 

 year into definite seasons. In other words, 

 this progressive change in the time of the 

 appearance of the eggs of gymnosperms 

 seems to hold some relation to the evolu- 

 tion of climate. It is significant, perhaps, 

 that the two great living groups of gymno- 

 sperms, Cycads and Conifers, are contrasted 

 not only in the feature under discussion, 

 but also in geographic distribution. The 

 Cycads, primitive in the late appearance of 

 eggs, are tropical; while the Conifers, ad- 

 vanced in the early appearance of eggs, are 

 found in the sharply differentiated seasons 

 of the temperate regions. In any event, wg 

 know that a gamete is a response to condi- 

 tions affecting unfavorably the ordinary 

 metabolism of a plant; and the most reg- 

 ularly recurring variable that affects nat- 

 ural vegetation is climate. 



2. The Proembryo. — A parallel illustra- 

 tion of progressive evolution is presented 

 by the proembryo of gymnosperms. Un- 

 fortunately the embryos of paleozoic gymno- 

 sperms have not as yet been found, not be- 

 cause they did not exist, as some have im- 

 agined, but because we have not been sec- 

 tioning the proper seeds. In any event, it 

 is now becoming safe to predict their gen- 

 eral character. 



In the most primitive gymnosperms the 

 proembryo is an extensive tissue, completely 

 filling the large egg, best illustrated by 

 Gingko among living forms. Just as in the 



