262 Journal of Agricultural Research voi. xviii. no. s 



been observed by which the pollen tube may gain access to the sheath 

 cells of the fibro-vascular bundles of the silk. Shortly after the pollen 

 tube appears, it may penetrate a hair and through it gain entrance to 

 the fibro-vascular bundle region (PI. 29, D) ; or the tube may continue 

 down the outside of hair to its base and then enter the silk and pene- 

 trate to the cells surrounding the bundle. Frequently pollen grains 

 that fall directly on the smooth portion of the silk germinate, and the 

 pollen tube penetrates the silk. These instances, however, are excep- 

 tions. Practically all pollen tubes that function are from pollen grains 

 that fall on the hairs of the silk. 



The end of the pollen tube is greatly enlarged as it pushes its way 

 between the dense sheath cells of the bundle (Pi. 29, E). In its passage 

 down the silk the tube causes but very little disturbance in the position 

 of the cells, so that after the tube disappears the cells quickly return to 

 their normal form and position. The pollen tube, so far as I have 

 observed, does not extend the full length of the silk at any time. It is 

 very difficult to locate it a short distance back of its growing region. 

 It appears that the older portions of the tube are absorbed by the sur- 

 rounding cells, while the growing part of the tube apparently is nourished 

 by the dense sheath cells. Arriving at the base of the silk, the pollen 

 tube pushes its way between the sheathlike cells that extend from the 

 bundle of the silk to the cavity of the ovary (Pi. 19, vsc). After it enters 

 the ovary cavity the tube twists and coils in its passage along the coats 

 of the ovule until it reaches the micropyle. After passing through the 

 micropyle, the tube works its way between the cells of the ovule and 

 enters the embryo sac (PI. 28, pt). The protoplasm of the pollen tube 

 is very dense, so that it is very difficult to locate the sperm nuclei. I 

 have never observed them in the tube except after it had entered the 

 embryo sac. 



If pollen is supplied abundantly, a great number of pollen tubes start 

 to grow down the bundle regions of each silk. However, as one examines 

 the silk from the tip downward, the number of pollen tubes becomes 

 smaller and smaller, so that when the cavity of the ovary is reached only 

 one pollen tube is to be observed. In nearly a hundred observations 

 no more than one pollen tube was seen in each ovary cavity. 



The growth of the pollen tubes is very rapid, and under ordinary 

 conditions they reach the embryo sacs of all the ovules on the ear in 

 24 hours after pollination. In order to do this the longest tubes must 

 grow in that time approximately 6 inches, a distance that equals 1,500 

 times the diameter of the pollen grain. 



