264 Journal of Agricultural Research voi. xviii. No. s 



way between the cells to the fibro-vascular bundles. Each silk has two 

 fibro-vascular bundles. These bundles are surrounded by sheath cells 

 which are characterized by their extremely dense contents and large, 

 flattened nuclei. It is between these cells that the pollen tube travels 

 down the silk. Arriving at the base of the silk, the pollen tube works 

 its way between the sheathlike cells that extend from the fibro-vascular 

 bundle of the silk to rtie cavity of the ovary. The tube enters the ovary 

 cavity and twists and coils in its passage along the ovule coat until it 

 reaches the micropyle. The tube then pushes between the cells of the 

 ovule until it reaches the embryo sac. The growth of the pollen tubes 

 is very rapid, so that they reach the embryo sacs of all the ovules of the 

 ear in 24 hours after pollination. To do this some of the tubes must 

 grow a distance of approximately 6 inches in the course of the 24 hours. 

 The pollen tubes apparently do not extend the full length of the silk at 

 any given time but are absorbed a short distance back of their tip by 

 the cells between which they pass. A great number of tubes start 

 down a given silk; but the number of tubes becomes less and less as the 

 base of the silk is approached, so that by the time the cavity of the ovary 

 is reached only one tube is to be observed. The two sperm nuclei are 

 formed in the pollen grain before the pollen tube appears. 



Fertilization. — The pollen tube enters the embryo sac and pushes 

 its way upward until its tip is near the polar nuclei. The tip of the tube 

 expands until it is approximately one-third the width of the embryo sac. 

 The wall of the tube seems to dissolve, giving the sperm nuclei access to 

 the embryo sac. One of the sperm nuclei fuses with the egg, and at about 

 the same time the other fuses with one of the polar nuclei. The two 

 polar nuclei fuse at the time the sperm nucleus enters one of them or 

 shortly afterwards. The pollen tube persists in the embryo sac until 

 it is crowded out by the developing endosperm and embryo. Fertili- 

 zation occurs in from 26 to 28 hours after the silks have been pollinated. 



EndospeJrm and embryo. — The endosperm nucleus soon divides, 

 and in from 10 to 12 hours after fertilization the endosperm nuclei may 

 number as high as 30, arranged around the periphery, of the embryo sac. 

 Within 36 hours after fertilization the cells of the endosperm completely 

 fill the embryo sac. The nucleus of the ferilized egg does not divide 

 for some time, so the endosperm may number 20 or more cells before the 

 first di\dsion of the egg takes place. When the cells of the endosperm 

 completely fill the embryo sac, the embryo numbers only 14 to 16 cells. 



LITERATURE CITED 



(i) Cannon, William Austin. 



1900. A MORPHOLOGICAL STUDY OF THE FLOWER AND EMBRYO OP THE WILD 



OAT, AVENA FATUA L. In Proc. Cal. Acad. Sci., s. 3, Botany, v. i, 

 no. 10, p. 329-364, pi. 49-53- Bibliography, p. 354-355- 

 (2) Crozier, a. a. 



1888. SILK SEEKING POLLEN. In Bot. Gaz., v. 13, no. 9, p. 242. 



