Study of Reproduction in the Genus Acer 115 



Floral Development 



The maples fall into two classes with respect to their time 

 of flowering, the one blooming before the development of the 

 leaves, the other after they have begun to expand. This fact 

 has been recognized in the subdivision of the genus (14). In 

 the region of Philadelphia Acer saccharinum and Acer rubrum 

 bloom within about ten days of each other, in late February or 

 early March. Then there is a period of ten days or more before 

 any other forms appear. These first two both flower before the 

 leaves appear, and so also does Acer negundo, which ushers in 

 the most active blooming period. A few of the exotic species 

 only, seem with us to delay beyond the latter part of April. 



The early blooming of the three forms named, correlated as 

 it is with a reduced perianth and marked dicecism, would lead 

 one to suspect a possible difference from other species in the 

 period of pollen maturation and other reproductive phenomena. 

 Only one of these, however, shows such a condition. Acer sac- 

 charinum, which flowers earliest in the spring, matures its pollen 

 in the autumn. The writer followed the development of the 

 flower buds closely during September and October, 1919, and 

 found that development was very gradual till the time for the 

 reduction divisions came near. Then a rapid growth in size 

 accompanied the divisions. This phase came about October 

 20-24. The period of the formation of the tube and generative 

 nuclei appears to be in late winter, generally during early Feb- 

 ruary (Fig. 27). Flowering occurs from the latter part of Janu- 

 ary, by the earliest records available, to the middle of March 

 in exceptionally late seasons and sporadic cases. The great 

 majority of the trees flower together during the first thoroughly 

 warm spell of the year. 



This species is followed by Acer rubrum, generally during the 

 early part of March, though, exceptionally, earlier. Here pollen 

 maturation does not take place in the fall, but during the open- 

 ing of the flower buds. The reduction divisions take place 

 during the swelling of the bud, and the tetrad stage is reached 

 as the anthers appear between the scales at the tip. The for- 

 mation of the tube and generative nuclei occurs just before the 

 rapid elongation of the filaments which accompanies anthesis. 

 The sperm nuclei were not seen: they are formed probably 

 subsequent to the shedding of the pollen. 



