64 MARGARET C. FERGUSON 



been approached by this pollen-tube had already been fertilized, 

 and the pollen-tube had turned aside and was passing up over 

 the top of the endosperm, as if seeking for another egg. The 

 position of the various elements of the pollen-tube is therefore 

 normal, the larger sperm-nucleus being in reality in advance of 

 the smaller. This suggests that, w^hen a pollen-tube has con- 

 jugated with the egg, a substance may be secreted which repels 

 other pullen-tubes, as has been described in case of spermato- 

 zoids in the Bryophytes and Pteridophytes. 



The formation of the sperm-nuclei shows most beautifully the 

 manner of the development of the nuclear reticulum. The 

 chromosomes unite end to end, giving rise to a homogeneous, 

 coiled band, before the nuclear membrane is formed. When 

 the nuclear-wall has been differentiated, the coil expands about 

 the periphery of the nucleus, while the band broadens, at the 

 same time becoming irregularly jagged along its margins. 

 These irregularities increase in length until finally those from 

 adjacent threads meet and fuse, thus giving rise to the reticulum 

 (figs. 107-110, plate X). When the sperm-nuclei have nearly 

 or quite come into contact they have as a rule reached their ma- 

 ture size. More than a year has now elapsed since pollination. 



Elongation of the Pollen-tube. — Up to this time the pollen- 

 tube has elongated very slowly, having penetrated as yet little, 

 if any, beyond the nucellar tissue of the previous year's growth. 

 In this upper portion of the nucellar cap the tube may become 

 very broad, or it may branch freely (figs. 71, 72, plate VI, and 

 83, 87, plate VIII). When the sperm-nuclei have attained their 

 full size, the downward growth of the tube is exceedingly rapid, 

 travelling in from eight to ten days more than twice the distance 

 traversed during the entire preceding year. The path pursued 

 during this rapid growth is comparatively straight and the tube 

 is unbranched (fig. 73, plate VII). In Pinus Strobus^ P. rigida 

 and P. mistriaca about ten days intervene between the division 

 of the generative nucleus and fertilization ; in Pinus ino7itana 

 unci^iata, the two processes are separated by an even shorter 

 space of time. 



The sperm-nuclei which at first present a very beautiful, rather 

 delicate reticulum (figs. 112, plate X, 117, plate XI), become 



