December 17, 1915] 



SCIENCE 



859 



= 0.62 cu. mm. calculated, = 0.63 cu. mm. 

 found. 



The rate of production of helium per 

 year per gram of radium ^163 eu. mm. 

 calculated, =164 cu. mm. found. 



B. B. BOLTWOOD 



A SUGGESTED EXPLANATION OF "OBTBO- 

 GENESIS" IN PLANTS 



The purpose of this paper is not to dis- 

 cuss what is called orthogenesis in plants in 

 general, but to cite certain notable illustra- 

 tions of it, and to suggest a possible expla- 

 nation. There may be some difference of 

 opinion as to the proper definition of ortho- 

 genesis, but it is used in this paper as stand- 

 ing for progressive evolution in a given 

 direction, in contrast with more or less suc- 

 cessful variations in several directions, in- 

 volved in the theories of natural selection 

 and mutation. 



My thesis is not to prove that ortho- 

 genesis differs in kind from such explana- 

 tions of. evolution as natural selection or 

 mutation, but that the persistent variation 

 which results in what is called orthogenesis 

 is in response to a persistent change in the 

 conditions of living. It is an explanation of 

 orthogenesis which contradicts its original 

 meaning and makes it a physical rather 

 than a vitalistic phenomenon. 



Another prefatory statement should be 

 made. The conclusions reached in this 

 paper are not simply inferences from a 

 series of observations, but are based chiefly 

 upon the results of experimental work 

 which indicates that the changes called for 

 can be induced as responses to changed 

 conditions. 



The gymnosperms are unique among the 

 great plant groups in the length of their 

 available history, recorded in such a way 

 that our knowledge of the group may be 

 said to be fairly continuous. Other great 

 groups are either relatively short-lived, or 



their records, at least so far as our knowl- 

 edge of them is concerned, are very discon- 

 tinuous. As a consequence, many lines of 

 advance among gymnosperms can be traced 

 in unbroken series from the Devonian to the 

 present time, involving structures that have 

 been assumed to be beyond the influences of 

 external conditions. I wish to call atten- 

 tion to four such lines of advance, and to 

 draw certain conclusions which have some 

 bearing upon evolutionary theory. 



1. The Egg. — A remarkable series of pro- 

 gressive changes is recorded as one traces 

 the development of the female sex organ 

 (archegonium) from the most primitive 

 gymnosperms to the most recent. The 

 gradual change consists in the shifting of 

 the time of appearance of the archegonium 

 in the ontogeny of the gametophyte (the 

 sexual individual). In the most primitive 

 gymnosperms the archegonia appear at 

 what may be called the full maturity of the 

 gametophyte, just as they do in the pro- 

 thallia of ordinary ferns. An unbroken 

 series can be traced, representing an earlier 

 and earlier appearance of archegonia in the 

 ontogeny of the gametophyte, extending 

 from full maturity to very early embryonic 

 stages. In this ontogeny three stages may 

 be roughly distinguished: (1) free nuclear 

 division; (2) primary wall formation; (3) 

 growth of tissue. It is toward the end of 

 the third stage that archegonia appear in 

 the most primitive gymnosperms; and the 

 gymnosperms of to-day, whose archegonia 

 are late in appearing, as the Cycads, are 

 primitive in this feature, though they may 

 be advanced in some others. 



As one proceeds with the history of the 

 group, it can be observed that the appear- 

 ance of archegonia shifts back through the 

 third stage, more and more tissue being 

 developed after their appearance. Next 

 they are observed forming at the second 

 stage, that of primary wall formation. In 



