316 BROOKLYN BOTANIC GARDEN MEMOIRS 



tree, the pistillate plant of this dioecious species has been proved to 

 mature its stigmas synchronously with the shedding of pollen from 

 the staminate trees. The yearly variations from the above dates 

 vary according to environal- — mainly heat — stimuli. Thus in 1912, 

 after a cold February and early March, succeeding favorable days 

 caused complete pollen dispersal on the 27th and in part on the 28th. 

 During the precocious springtime of 191 3, the flowering occurred 

 suddenly and uniformly on the morning of March 18, while in the suc- 

 ceeding year like expansion took place on March 31. The Carolina 

 poplar {Populus deltoides) is like the last species specially abundant 

 in the staminate trees, rare in the pistillate ones. Its average bloom- 

 ing period is April 16. In 1913 its behavior was arresting in the sud- 

 den and exact procedure shown. Here, these trees under similar 

 environment lengthened their catkins fully and started to disperse 

 abundant pollen on March 24, between 9 and 10 A.M. of a bright 

 warm day. The young catkins were almost emptied by 5 P.M., 

 only a few of the smaller terminal flowers still retaining a quantity. 

 By next day, scant remnants could alone be secured. But on that 

 day four trees which grew on wind-swept and shaded street corners 

 were found only beginning to dehisce. 



In 1914, owing to prolonged cool periods and warmer ones alter- 

 nating, blooming occurred on April 6. Many catkins matured only 

 on the 7th. During the present year, the action and reaction of 

 environal energy and of organismal tissue have been most suggestive. 

 For stimulated by the bright warm suns of March 31 and April i 

 some catkins lengthened gradually during these and succeeding days, 

 and began to discharge pollen extensively on April 4. But rather 

 low temperatures on that day and those succeeding, accompanied 

 often by rain, prolonged reaction fitfully until the loth of the month. 

 So we learn from such statistics, that have frequently been verified 

 for the above, as well as for other species, that if a sufficient environal 

 stimulus act quickly and continuously, an extensive synchronous 

 blossoming may ensue, that is completed within a few hours each year. 

 On the other hand, if temperature units be more gradually expended, 

 and specially if such be combined with wetting conditions that prevent 

 establishment of tissue tension in anthers, pollen discharge may be 

 prolonged over a considerable period. 



The alder (Alnus incana) that averagely lengthens and opens its 

 catkins on March 24, and that shows crimson papillose stigmatic 

 surfaces in exactly synchronous manner, was in like state on the 25th, 

 but in 1913, March 12 was the date. Pollen dispersal is usually com- 

 pletely effected in three or at most four days, this lengthened period 

 being due to differences in position and so in time maturation of the 

 catkins on each twig. 



