sels, found in inbred P89, are partly formed by tissues 
from the main axis. Such modifications in structure are 
imposed by the different conditions of development in 
tassel and ear and do not detract from the important 
anatomical evidence of a close similarity between the 
lining of the cupule and the pulvinus. 
The cupule lining and pulvinus as homologues of the 
prophyll. The pulvinus resembles a rudimentary prophy]I 
in phytomeric position, in external appearance and in 
internal structure. The cell structure of the pulvinus 
suggests the primordium of an organ, such as the pro- 
phyll, in that the swelling results from a proliferation of 
many small cells rather than from an expansion in size 
of individual cells. It is rudimentary in development in 
being delimited by a zone of rapidly changing cells rather 
than by an abrupt boundary of cells such as might occur 
between elaborated organs which are fused (Plate V). 
Inasmuch as the anatomy and phytomeric position of the 
pulvinus is also similar to that of the cupule lining, as 
discussed previously, we conclude that these formations 
are different manifestations of the rudimentary prophyll- 
part of the phytomer. Therefore, it is necessary to mod- 
ify our previous conception of the cupule lining as an 
adnate prophyll (Nickerson, 1954; Galinat, 1956) to the 
extent that we now believe that the cupule lining is only 
one of several possible manifestations of the rudimentary 
prophyll-part of the phytomer. Under other develop- 
mental conditions elsewhere in the plant, the tissue from 
this region has ultimately developed in the form of a 
prophyll, a pulvinus or a palea. 
The role of pressure in cupule formation. The effect of 
pressure from constricting leaf sheaths upon floral devel- 
opment and floral evolution in the grasses has been rec- 
ognized (Arber, 1934, and others). In maize, such pres- 
sure moulds expansion of a plastic inflorescence from the 
[ 23 | 
