244 Causes and Course of Organic Evolution 



of energy mainly characterize the process; these being in turn 

 very dehcately regukited by environal conditions. Thus not 

 only does each species of Spirogyra conjugate at definite sea- 

 sons, the cells in opposite filaments mature within a few hours 

 or days apart each year. The same is true for gatherings of 

 desmids, diatoms, and related algse. In Oedogonium, Nitella, 

 and Chara definite growth-waves and maturation-waves of 

 energy traverse the plant from below upward, till the motile 

 sperms have been shed, and the eggs have been fertilized. 



In hepatics and mosses generally, the maturation of anther- 

 idia and archegonia proceeds within surprisingly exact time 

 limits, as does also that of spore formation in alternating rela- 

 tion. In ferns the period for maturation of the antheridia 

 and archegonia on each prothallus can be very accurately 

 predicted for each species, if note be taken of light, moisture, 

 and temperature conditions. 



But it is amongst the flowering plants that these phenomena 

 are most pronounced, though this is probably due to such 

 being most readily observed and conspicuously shown. The 

 writer has noted, for about 14 years, the comparative flowering 

 period of Ginkgo biloba round Philadelphia, and there for sev- 

 eral staminate and pistillate trees some distance apart. While 

 the period of blooming has been as early as April 25th during 

 warm seasons and as late as May 20th during cold seasons, the 

 staminate and pistillate trees have been synchronous often 

 •to a day, while anthesis and pollination have largely been 

 limited to about four hours, namely between 7 and 11 a. m. 

 of one day. 



Equally exact results have been recorded for the staminate 

 and pistillate trees of the white poplar (P. alba), the Carolina 

 poplar, (P. monilifera), and subdioecious trees of the silver 

 maple (Acer saccharinum) as also for the hermaphrodite sho^^'y 

 flowers of the catalpa tree (C bignonioides), and many others. 



But the views regarding, and mode of presenting, the ques- 

 tion of sex differentiation are often as varied as they are illog- 

 ical. For in not a few evolutionary treatises that discuss the 

 subject, instead of any patient attempt being made to start 



