2o6 MORPHOLOGY OF GYMNOSPERMS 



THE MALE GAMETOPHYTE 



The first account of the male gametophyte of Ginkgo is that given 

 by Strasburger (15) in 1892. It is of interest to note that Hof- 

 MEiSTER in 1863 (i) predicted the finding of swimming sperms in 

 the pollen tubes of gymnosperms; and in May 1896, at a meeting 

 of the Botanical Society of Tokyo, Hirase announced (18) the dis- 

 covery of ciliated sperms, his preliminary paper (19) appearing in 

 October of the same year, and his full paper (22) in 1898. In this 

 connection a detailed study was made of the development of the male 

 gametophyte and of the phenomena connected with fertilization; 

 and it is chiefly from this source that the following account is drawn. 



The first division of the microspore is unequal, resulting in cutting 

 off a small lenticular cell, which soon disorganizes and later appears 

 like a cleft in the heavy spore wall. No such ephemeral vegetative 

 cell has been discovered among the Cycadales, but such cells are 

 characteristic of the Abietineae. 



A second unequal division occurs, and a second small cell is cut 

 off, which persists, and which is a second vegetative cell. This cell 

 also appears among the Abietineae, but in them it is ephemeral like 

 the first. A single persistent vegetative cell also appears in the Cyca- 

 dales; so that Ginkgo resembles the Abietineae in having two vege- 

 tative cells, and the Cycadales in having one persistent cell. 



After the two vegetative cells have been cut off, the remaining 

 large cell may be regarded as the antheridium initial, although the 

 ordinary structure of a multicellular antheridium has disappeared. 

 This cell divides very unequally, the smaller cell, in contact with the 

 persistent vegetative cell, being the generative cell, and the larger cell 

 being the tube cell (fig. 235). The gametophyte is in this four-celled 

 stage (ephemeral vegetative, persistent vegetative, generative, and 

 tube cells) when pollination takes place (fig. 236), during the last of 

 April or early in May (in Japan). 



The pollen chamber now begins to enlarge at the expense of the 

 nucellar tissue at the side and beneath, carrying the grains deep into 

 the nucellus. Early in June the chamber has enlarged greatly, form- 

 ing a large irregular cavity, which extends nearly to the embryo sac. 

 When the grains begin to put out their tubes, the opening to the pollen 

 chamber becomes closed and remains so, the surrounding tissue 



