responsible for the initiation of permanent grooves in the 
internodes of the vegetative axis (Arber, 1934; Ander- 
son, 1949). Similarly, there is evidence that compression 
is a factor in the depression of rachis internodes. During 
early floral development in maize, the original branch 
primordia, each of which is the initial for a pair of pis- 
tillate spikelets, are compressed against the rachis with 
the plane of the branch diverging from the plane of the 
rachis at about a 45° angle (Kiesselbach, 1949, Fig. 38; 
Bonnett, 1940). Subsequent expansion of the differenti- 
ating spikelets without a corresponding elongation of the 
rachis-segments, forces the spikelets downward to a 90° 
angle, while a cupule appears in the rachis at their former 
resting place. 
The role of compression in the formation of this cupule 
is revealed by the effects of certain genes in altering the 
usual scheme of ontogeny. Thus, when the pistillate 
rachis is free from compression during youth, as in the 
upper portion of the ears of certain tunicate varieties or 
corn-grass strains, then the cupules are shallow and the 
rachis-flaps are relatively inconspicuous. Also, the physi- 
cal nature of the pistillate spikelets influences the depth 
of cupules. For example, if the spikelets are papery 
(papyrescent maize), the cupules are shallow and if the 
spikelets are borne singly (corn-grass maize), the cupules 
are narrow. But if, on the contrary, the spikelets are 
corneous and remain appressed to the rachis (as in certain 
highly ‘“Tripsacoid’’ varieties), then the depressions are 
deep and the rachis-flaps are prominent. 
The adnate prophyll. The cupule is more than just a 
depressed rachis-segment. Cutler and Cutler (1948) have 
noted that it seems to resemble a small confined leaf 
whose lateral auricles are the ‘‘rachis flaps’” or cupule 
wings and the subtending auricle notches are the ‘‘hairy 
notches’’ of the cupule. More recently, Nickerson (1954) 
[ 228 | 
