Dec. 1, 1919 Pistillate Spikelet and Fertilization in Zea mays L. 259 



of the tip of a young cob in longitudinal section (Pi. 21, A) differs a little 

 from that in cross section (Pi. 20, A). 



A longitudinal section of the rudiment of a spikelet just after its appear- 

 ance shows that it is composed of embryonic cells and has the general 

 appearance of the tip of the young cob. The first differentiation to appear 

 on the rudiment of the spikelet is the lower empty glume (Pi. 21, B). 

 The primordium of the upper empty glume soon appears (Pi. 2 1 , C) , so that 

 at a little later stage the two developing glumes have practically the same 

 appearance (Pi. 21, D). The primordia of the tvv^o lemmas or flowering 

 glumes are the next to appear (Pi. 22, A), while directly following, or fre- 

 quently at the same time, the rudiments of the sterile flower and of the 

 stamens of the fertile flower become visible. The palet of the fertile 

 flower at this time also begins to show differentiation (Pi. 22, B), but the 

 palet of the sterile flower does not appear until considerably later. 



The primordium of the carpellate leaf or ovary wall of the fertile pistil 

 begins to show in a short time after those of the palet and stamens of the 

 fertile flower appear (Pi. 23, A). At this time the cells that are to com- 

 pose the fibro-vascular bundles of the lower part of the spikelet begin to 

 differentiate. The carpel grows unevenly so that when the side adjacent 

 U) the lemma extends almost one-third around the young ovule the 

 opposite side has scarcely begun to develop (Pi. 23, A, c, c'). This more 

 rapidly growing portion of the carpel increases in width toward the tip 

 so that it becomes from two to three times wider than the base (Pi. 23, B). 

 This wddened portion of the carpel is composed of numerous embryonic 

 cells which later rapidly elongate to form the silk (Pi. 24, A). When the 

 silk is elongating, the wall of the ovary has grown up around the ovule 

 and has almost inclosed it with the exception of a small opening toward 

 the top (Pi. 24, A, sc). This opening has been termed the stylar canal. 

 It, however, does not long remain open, for by the time the silk is ready 

 for pollination, the edges of the carpel have come in close contact but 

 have not grown together (Pi. 25, B). 



About the time the silk begins to elongate, the ovule begins to invert. 

 The cells of the ovule on the side adjacent to the palet increase in number 

 and elongate more rapidly than those on the opposite side, thus causing 

 the end of the ovule to turn downward (Pi. 24, B). The megaspore 

 mother cell appears about the time the ovule begins to turn, and fre- 

 quently the embryo sac has reached the 2 -celled stage by the time the 

 ovule has become completely inverted. The ovule coats grow rapidly 

 when the ovule begins to curve, so that by the time it has reached its 

 final position they have reached their full development (PI. 25, A). 



DEVELOPMENT OF THE EMBRYO SAC 



About the time the ovule begins to invert, the differentiation of the 

 megaspore mother cell becomes apparent (PL 24). No disorganization 

 of any of the megaspores was noted in the three varieties of corn studied 



