136 THE STRUCTURE OF ECONOMIC PLANTS 



The midribs of the bundles of the lateral carpels extend into the 

 silk, their marginal bundles supplying the ovule. The third carpel, 

 which is regarded as rudimentary, has a vestigial median bundle 

 that is sometimes present at the base of the ovary opposite the point 

 of divergence of the ovule. The nucellus arises in an erect position; 

 and, shortly before the megaspore mother cell is differentiated, 

 there is an acceleration of cell division and growth on the side of 

 the nucellus adjacent to the palea. As a result of this differential 

 growth, the ovule finally becomes semianatropous or approximately 

 campylotropous. There is no well-defined funiculus at the broad 

 base of the ovule, and the two integuments which are initiated at 

 this point are free from each other. The inner integument com- 

 pletely surrounds the nucellus except at the micropyle; but the 

 outer one extends only partially around it, forming a wedge- 

 shaped angle where it projects into the funnel-like depression of 

 the stylar canal. (Fig. 56-^.) 



The Megagametophyte. — Megasporogenesis occurs at about 

 the time that the ovule begins to curve. The megaspore mother 

 cell is then differentiated and subsequently four megaspores are 

 produced, the innermost one being functional. When the mega- 

 gametophyte reaches the eight-nucleate stage, two polar nuclei 

 move to the center of the embryo sac; but they do not completely 

 fuse prior to fertilization. The three antipodal nuclei begin to 

 divide at once, and the number increases until as many as 14 to 36 

 cells are formed which may be binucleate. Meanwhile, the 

 synergids become elongated or lunar-shaped with very dense con- 

 tents. In most instances, they do not remain intact after the time 

 for fertilization has arrived and usually disintegrate prior to that 

 event. The megagamete increases in size until its width is approxi- 

 mately half that of the embryo sac. 



The Microgametophyte and Pollination. — The development 

 of the stamens is like that of other grasses; and, following reduc- 

 tion division, the pollen matures to form grains that are almost 

 spherical, with a minutely roughened exine and a prominent germ 

 pore. A thickened ring of the intine surrounds the pore and closes 

 it with a plate of tissue that resembles the rest of the wall in struc- 

 ture. The pollen grain, now a microgametophyte, has dense 

 protoplasm and contains a vegetative and a generative nucleus. 

 Before shedding, the latter divides to form two long, slender 

 microgametes that are crescent-shaped and pointed at the ends. 



