64 postelsia 



four perfect cotyledons, as for example 

 Acer. 



The production of an apparently 

 dicotyledonous embryo through the 

 bifurcation of the originally single coty- 

 ledon of Nelumbo, has suggested that 

 this might have been the method of 

 origin of the dicotyledonous habit. In 

 Nelumbo the cotyledon is at first a 

 crescent shaped mound of tissue run- 

 ning around the rear upper surface of 

 the embryo. The plumule arises in 

 the sinus between the points of the 

 crescent. The pseudo-cotyledons later 

 arise through localization of growth so 

 that ultimately they occupy positions 

 on either side of the plumule as in a 

 truly dicotyledonous embryo. The ces- 

 sation of growth in the median line, 

 and its localization in the wings of the 

 cotyledon, take place at an early stage 

 in the embryogeny. The transition 



