Study of Reproduction in the Genus Acer 117 



flowers, opening later than the others, in which the ovaries 

 were normal in appearance. 



Details of abnormalities in the structure of individual flowers 

 were not noted, except the occurrence of tri-carpellary ovaries. 

 These were found in Acer rubrum, Acer saccharinum, Acer sacc- 

 harum, Acer platanoides, and Acer pseudo-platanus, the forms 

 in which quantities of female material underwent close exam- 

 ination. 



Pollen Development in Acer Negundo 



The early stages in the development of the anther and the 

 origin of the tapetum and archesporium were not traced. There 

 was some variation between the different species in the number 

 of layers forming the wall. An epidermal layer, and an outer- 

 most endothecial layer of larger cells were always present. In 

 addition inner wall layers were found, varying in number from one 

 which disorganized at maturity in Acer spicatum Lam. to three, 

 of which one persisted in the mature anther, in Acer pseudo- 

 platanus. The tapetum was always strikingly developed. It 

 was found to show a multinucleate condition of the cells in all 

 of the forms studied, and the nuclear divisions giving rise to this 

 state occurred both by a somewhat abnormal looking mitosis 

 and by amitotic fission. The latter was by far the commonest, 

 and the daughter nuclei often remained attached to each other, 

 forming a cluster near the center of the cell. Both began about 

 the time of synapsis, and in the mitotic type the number of 

 chromosomes appeared in excess of the normal sporophytic 

 number (Figs. 31-33). 



The material at hand has permitted a fairly complete study 

 of five species with regard to the details of the reduction divi- 

 sions in pollen formation. These are Acer negundo, Acer rubrum, 

 Acer platanoides, Acer pseudo-platanus and Acer saccharum, in 

 addition to which fragmentary material of several other forms 

 was available. Of these, by far the best quality of material 

 came from the first named. The low number of chromosomes 

 in this species makes it a very satisfactory one for study, and 

 since the writer found some differences of detail from the descrip- 

 tion given by Mottier (13) a synopsis of the stages in Acer ne- 

 gundo will precede a comparison with the other forms studied. 



