Study of Reproduction in the Genus Acer 133 



hypocotyl. The small lobe at the base of the third cotyledon 

 may or may not represent a partial development of the same 

 kind. 



Summary 



The Maples studied, with one exception, Acer saccharinum, 

 mature their pollen during the expansion of the buds in the 

 spring. Acer saccharinum matures its grains to the one nucle- 

 ate stage in the autumn, the division to form the tube and gen- 

 erative nuclei generally being delayed till shortly before flower- 

 ing. It is the first of the genus to bloom, being followed at inter- 

 vals by the other native forms, the exotic species closing the 

 series about the end of April. 



Pollen development in Acer negundo proceeds much as de- 

 scribed by Mottier, and in the other Maples studied the process 

 is similar. The haploid chromosome number is thirteen. Hap- 

 loid counts were made in Acer platanoides showing eleven, in 

 Acer saccharum showing thirteen, in Acer pseudo-platanus show- 

 ing twenty-six, in Acer saccharinum showing twenty-six, and 

 in Acer rubrum showing approximately seventy-two, fifty-four 

 and (megaspore mother cell) thirty-six chromosomes. 



Somatic counts were made in the root-tips of some species 

 with the following results: Acer platanoides, twenty-six; Acer 

 pseudo-platanus, fifty-two; Acer saccharinum, fifty-two, with also 

 isolated cells containing about twice that number; Acer car pinifo- 

 lium, fifty-two; and A cer rubrum, above ninety. The difference be- 

 tween the somatic and reduced counts in Acer platanoides mav be 

 due to differing strains with differing nuclear composition. The 

 conditions in Acer rubrum seem to indicate that a tetraploid 

 form exists, and the intermediate counts similarly seem to indi- 

 cate that hybrids with the normal diploid form occur. 



Ovule development was followed in Acer rubrum and other 

 species, with in the species named a row of three potential 

 megaspores resulting from the divisions of the mother cell. The 

 chalazal one persists, the other two degenerate. The persistent 

 cell gives rise to the embryosac, which is normal with eight 

 nuclei. The polar nuclei fuse before fertilization and the endo- 

 sperm nucleus passes to the middle of the sac before dividing. 

 The embryosac enlarges greatly, especially in Acer platanoides, 

 and many free endosperm nuclei are produced, but no cell walls 

 are laid down. The growth of the embryo does not accompany 



