GNETALES— WELWITSCHIA 401 



cones without effecting pollination. But whether the insect is the 

 principal agent in pollination may still be doubtful. It is very prob- 

 able that in most wind-pollinated plants, even in gymnosperms, 

 pollen may occasionally reach the poUination drop on the stigma 

 through the agency of insects. 



Reaching the poUination drop in the three-nucleate stage, the 

 pollen is drawn down through the long micropyle to the nucellus 

 of the ovule. The pollen tube may begin to develop, even in the 

 micropyle. There is no pore, but the exine spHts longitudinally 

 throughout the entire length. 



In the young pollen tube the generative cell and the tube nucleus 

 enlarge, and the prothallial nucleus can no longer be distinguished. 

 There is no stalk cell. The nucleus of the generative cell soon di- 

 vides, forming the two sperm nuclei, which lie in a common mass of 

 protoplasm, with no wall between them. 



Except for the prothallial nucleus, this is a typical angiosperm 

 history, and even the prothallial cell occurs sporadically in angio- 

 sperms. 



THE FEMALE GAMETOPHYTE 



The megaspore is the first cell of the female gametophyte. The 

 divisions of the megaspore mother-cell have not been worked out, 

 and no chromosome counts have been made anywhere; but Pear- 

 son's figures show that the archesporium was doubtless hypodermal 

 and that the lowest megaspore of a row germinates. He figures the 

 mature megaspore, the four-nucleate stage of the female gameto- 

 phyte, and later free nucleate stages. The nuclear divisions are 

 doubtless simultaneous, for simultaneous divisions were observed 

 at two stages. A striking feature of the development of this gameto- 

 phyte is that no large central vacuole is formed, the nuclei being 

 equally distributed throughout, up to what would be the 1,024 nu- 

 clear stage, if all divisions were simultaneous and all nuclei divided 

 (fig. 377). Actual calculation placed the number between 1,015 and 

 1,360. 



At the close of the free nuclear period, walls come in irregularly, 

 often inclosing a dozen nuclei in one cell. In this condition, there 

 are occasional nuclear divisions, but nuclear fusions are the rule 



