134 THE STRUCTURE OF ECONOMIC PLANTS 



also occur where the pericarp extends only partially between the 

 kernels. 



In the mature carpellate spikelet, like the staminate one, the 

 two flowers are enclosed by a pair of glumes and each flower is 

 subtended by a lemma and palea, these being much shorter than the 

 glumes. The two paleas are back to back and separate the flowers. 

 The sterile (lower) flower has a rudimentary carpel, three rudimen- 

 tary stamens, and two well-developed lodicules which lie outside of 

 and alternate with the stamens. The fertile (upper) flower has a 

 well-developed carpel, and rudimentary stamens can be distin- 

 guished in the early stages of ontogeny. The lodicules of the fertile 

 flower are not easily recognized when the spikelet is mature. The 

 palea and lemma of the lower abortive flower persist and with those 

 of the fertile flower constitute the chaff found at the base of the 

 mature grain. 



The sequence of development of the parts of the carpellate 

 spikelet is much like that of the staminate. From the primordium, 

 the lower glume, upper glume, and lemmas of the two flowers are 

 developed in the order named. These are followed closely by the 

 rudiments of the sterile flower and then the rudimentary stamens of 

 the fertile flower appear. The palea of the fertile flower next 

 differentiates and the palea of the sterile flower arises somewhat 

 later. 



Following the appearance of the palea and rudimentary stamens 

 of the fertile flower, the carpellary primordium of that flower 

 begins to develop. (Fig. 56.) This arises as a ring from the 

 rounded mass of meristematic tissue which remains after the 

 divergence of the staminal primordia. Growth of this ring is 

 uneven, the side adjacent to the lemma growing rapidly so that it 

 may extend one-third of the way around the young nucellus before 

 the opposite side has begun to develop. The more rapidly growing 

 portion of the ovary wall becomes thicker and more meristematic 

 at its top; and, from this region, the silk or style immediately 

 begins to develop. During the elongation of the style, the ovary 

 wall continues to grow until it completely encloses the ovule except 

 for a small opening at the top of the ovary and adaxial to the style. 

 This opening, which Guignard (7) termed the "stylar canal," later 

 closes as its edges come in contact. These do not unite, however, 

 and the canal can be recognized in the mature ovary. 



The silk is solid, somewhat flattened, with two grooves running 



