

CHAPTER XV 

 CONIFEROPHYTES— CONIFERALES— Cow/mMe^f 



FERTILIZATION 



The pollen tubes press and digest their way through the nucellus, 

 destroying much of its tissue; but there is no such complete destruc- 

 tion as in the cycads, where the pollen tubes hang free for a greater 

 part of their length. In conifers, the 

 pollen tubes reach the megaspore 

 membrane, with both gametophytes 

 in various stages of development. 



In Torreya taxifolia^^^ the pollen 

 tube completes its passage through the 

 nucellus while the female gameto- 

 phyte is still in an early free nuclear 

 stage, with only i6 or 32 free nuclei 

 (fig. 325), At this stage, the pollen 

 tube has broadened at the base, and 

 the body cell has enlarged, but will 

 enlarge much more before division. 

 Just as the pollen tube is about to dis- 

 charge its two very unequal sperms, 

 it is broader than the egg (fig. 326). 

 It should be noted that the interval 

 between the stages shown in these 

 figures is about 7 weeks. 



In Callitris verrucosa'^^'' the pollen 

 tube arrives when the female gameto- 

 phyte is in the 128 nucleate stage. 



In Taxus canadensis^'^^ the female 

 gametophyte is more advanced when 

 the pollen tube reaches it, and is cellular throughout, but the 

 archegonium initials are not yet recognizable (fig. 324). The body 



Fig. 324. — Taxus canadensis: 

 nucellus showing seven pollen 

 tubes, three of which show the 

 body cell just ready to divide. The 

 female gametophyte has not yet 

 reached the archegonium initial 

 stage; X92. — After Dupler.''* 



