CONIFERALES 95 



and enlarges, and presently becomes indistinguishable among the 

 numerous tortuous sterile megaspores which cluster about the 

 upper end of the elongated fertile one. The branches of the 

 tube which remain external to the nucellus advance in every 

 direction between it and the integument. Such a condition of 

 things suggests the free branching rhizoidal habit of the tubes 

 of Cycads and Ginkgo, although in them the branching is within 

 the body of the nucellus. See Appendix. 



FERTILIZATION 



The tip of the pollen tube, having penetrated the overlying 

 tissue of the nucellus, reaches the wall of the embryo sac, and 

 either passes directly through it or flattens out upon it in a foot- 

 like expansion, sending out a small branch to pierce the wall. 

 In any event, the tube, or a branch of it, reaches the neck of the 

 archegonium, crushes the neck cells, and comes in contact with 

 the egg. 



This contact may be regarded as the beginning of the pro- 

 cess of fertilization, which ends with the fusion of the sexual 

 nuclei. Jager 40 reports that in Taxus baccata the tube reaches 

 the egg about June 1st ; Coker 45 states that the date for Tax- 

 odium distichum is about June 15th ; the same date is given by 

 Arnoldi 43 for Cephalotaxus Fortunei\ while in Pinus Laricio 

 we find the date to be about July 1st. Of course the dates vary 

 somewhat with the season and with the latitude. 



It has been shown by Blackman 37 and confirmed by Cham- 

 berlain 39 that the tip of the tube becomes fused with the egg 

 membrane, and does not enter the cytoplasm, as usually stated. 

 The appearance of an elongated and well-defined cavity in the 

 cytoplasm of the upper part of the egg is due to the inrush of the 

 contents of the tube (Figs. 74, 76, B). 



The pit in the tip of the tube was described by Hofmeister, 5 

 Strasburger, 15 and others, but they claimed that it remains closed 

 by the primary wall membrane. In 1883 Goroschankin 26 stated 

 that he saw the pit perforated in Pinus Pumilio, which was 

 fully confirmed by Dixon 33 in Pinus silvestris, and by Black- 

 man 37 in the same species (Fig. 71). Nearly the whole of the 

 contents of the tube is injected into the cytoplasm of the egg. 

 In Pinus Pumilio , Goroschankin 26 observed both male cells pass 

 in, and Strasburger 28 confirmed it for Picea vulgaris. Dixon 33 



