1915] 



COULTER — ORIGIN OF MONOCOTYLEDONY 



183 



is really the space between the two very unequal cotyledons, 

 which surround the real apex of the embryo. The apex of the 

 embryo is at the bottom of the notch, and not at the tip of the 

 large embryo. This apex soon begins to form leaves, and 

 the so-called stem tip appears issuing from the bottom of the 

 notch, in a relation apparently lateral only because the two 

 cotyledons are so unequal. Furthermore, when the stem tip 

 is examined, it is found not to be a stem tip, but a cluster of 

 leaves whose rapid development has aborted one of the grow- 

 ing points on the cotyledonary zone. All this is very obvious 

 in grasses, and is equally obvious in any massive proembryo, 

 but it escaped the earlier observers of filamentous proembryos. 



The general conclusion is that monocotyledony is simply one 

 expression of a process common to all cotyledony, gradually 

 derived from dicotyledony, and involving no abrupt transfer 

 of a lateral structure to a terminal origin. 



This paper was prepared in collaboration with Dr. W. J. 

 G-. Land, who also supplied the material and made the illus- 

 trations. 



