Dec. 1. 1919 Pistillate Spikelet and Fertilization in Zea mays L. 261 



fertilization is delayed they lose their identity and can not be distinguished 

 from the surrounding cytoplasm. 



The polar nuclei are embedded in a strand of cytoplasm that extends 

 from the antipodals to the egg, while the greater part of that portion of 

 the embryo sac is taken up by two large vacuoles. The nucleoli of the 

 polar nuclei are the largest in the embryo sac. The two polar nuclei 

 remain in close contact but do not fuse until fertilization has taken 

 place. 



SIIvK AND THE POI^IvEN TUBE) 



The end of the silk is cleft into two branches of unequal length. This 

 branched portion of the silk has been termed the stigma by most authors 

 in their description of the corn flower (PI. 29, A). The silk, however, is 

 receptive to pollen for at least the greater portion of its length ; so it would 

 appear that Weatherwax (16) is correct in asserting that the term 

 stigma can be applied to the branched tip of the silk only in a morpho- 

 logical sense and not with the understanding that it is the only portion 

 of the pistil on which the pollen grains may germinate. 



Numerous hairs are borne on the silk in rather definite areas for its 

 entire length (PI. 29, A). These hairs appear for the most part on the 

 edges of the silk and are more numerous near its tip than farther down. 

 The hairs may be branched or unbranched and the upper ends of the 

 cells that compose them stand out from the hair (PL 29, B), thus form- 

 ing a rough surface upon which the pollen grains easily lodge. The origin 

 and development of these hairs have been described in detail by Weather- 

 wax (13), who observed that each hair originates from a single epidermal 

 cell of the silk. 



Two fibro-vascular bundles extend the entire length of the silk and 

 terminate in the branched tip (PI. 29, A). A cross section of the silk 

 shows that it is grooved on both its upper and lower surfaces and that 

 the vascular bundles are located near its edge (PI. 30, A). Each bundle 

 contains from three to six xylem elements (PI. 30, B). The conducting 

 tissue of the fibro-vascular bundles is surrounded by narrow, elongated 

 cells that are characterized by very dense cytoplasmic contents and 

 elongated flattened nuclei (PI. 30, C). It is between these dense cells 

 that the pollen tube travels down the silk. 



The pollen grains vary in shape from spherical to ellipsoidal, and 

 each grain has a germ pore (PI. 29, C). The protoplasm of the pollen 

 grain is very dense, and often it is difficult to distinguish the nuclei. 

 The two sperm nuclei are formed before the pollen is shed (PI. 32, A). 

 This supports the statement of Strasburger (ij) that the division of the 

 generative nucleus in the pollen grain is a constant character for all the 

 grasses. 



A few hours after the pollen grains lodge on the hairs of the silk, the 

 pollen tube emerges from the germ pore (PI. 29, D). Three ways have 



