334 GYMNOSPERMS 



and Ihc archcgonium is in a rather advanced stage of development 

 before the pollen tube comes into contact with the megaspore 

 membrane. 



In any case, the division of the body cell to form the two sperms 

 takes place shortly before fertilization, so that it is more or less simul- 

 taneous with the division of the central cell to form the egg and the 

 ventral canal cell, or ventral canal nucleus. 



The neck cells in many cases disorganize, and the megaspore 

 membrane at the top of the female gametophyte is dissolved or be- 

 comes so weak that it is easily broken when the pollen tube arrives. 



When the pollen tube reaches the egg, the behavior differs. In 

 the Abietaceae the tip of the tube ruptures, forming a pore through 

 which the contents are discharged into the egg. The pollen tube 

 itself does not enter. 



In the other families more or less of the tip of the pollen tube 

 enters the top of the egg before the discharge takes place. Many of 

 the observations upon which this statement is based should be con- 

 firmed or corrected, for there is, at the time of the discharge, a large 

 vacuole at the top of the egg, and the vacuole often has such a strong 

 plasma membrane that it might be mistaken for the end of a pollen 

 tube. There have been occasional misinterpretations of pollen 

 tubes ever since they were thought to be embryos. 



The pollen tube, when it comes into contact with the egg, has been 

 growing more and more turgid, so that, when the rupture comes, the 

 contents of the tube are discharged with considerable violence. 

 Some forms, like Pinus, have been studied so thoroughly by so 

 many investigators that anything exceptional is easily distinguished 

 from the normal behavior. A few other forms have been studied al- 

 most as thoroughly by several investigators. Unfortunately, many 

 of the genera have been described by a single investigator in a single 

 paper, and the investigations have too often been based upon exotic 

 material or material growing naturally, but collected and sent to 

 the investigator from a considerable distance. The variations which 

 are known to occur in Pinus should be borne in mind while weighing 

 accounts based upon comparatively limited material. 



In Pinus both sperms, together with the stalk and tube nuclei 

 and some of the protoplasm, are discharged into the egg. In Taxus 



