CYCADALES 129 



claim that there is any insect pollination should be supported by the 

 most critical field study. 



At the time of pollination a large pollination drop appears at the 

 micropyle. Cells at the top of the nucellus break down and some of 

 their contents ooze out as a mucilaginous drop. The pollen grains 

 fall on the drop and, as it dries, are drawn into the young pollen 

 chamber below. Further drying seals the chamber, and the top be- 

 comes very hard, forming the nucellar beak. 



Pollen germinates readily in sugar solutions, in thick juice of pear 

 preserves, and in many syrups. The pollen tube soon appears, and 

 grows to several times the length of the pollen grain, but the genera- 

 tive cell does not divide. Cultures kept for a month show no division ; 

 but in material taken in the field, probably not more than a week 

 after pollination, the division has taken place, giving rise to a stalk 

 cell and a body cell, the name "stalk cell" being given from a sup- 

 posed homology with the stalk cell of the antheridium of Pterido- 

 phytes. 



The pollen tube, coming from the upper end of the pollen grain, 

 grows into the nucellus, and acts as a haustorium, conveying food 

 material to the basal end of the tube. In Ceratozamia, in addition to 

 the usual haustorial pollen tube, there are numerous haustoria ex- 

 tending downward from the base of the pollen tube. The genus could 

 be identified by this feature as positively as by the two horns on the 

 sporophyU, which give it its name. The pollen tube is a haustorium, 

 not a sperm-carrier as in angiosperms. 



There is no further cell division for a long time, the division of the 

 body cell taking place almost immediately before fertilization. The 

 interval between pollination and fertilization is about four months in 

 Cycas revoluta, five months in Zamia floridana, and about six months 

 in Dioon edule. 



During this long period the pollen tube digests its way downward, 

 enlarging the pollen chamber until it finally extends completely 

 through the nucellus and the pollen tubes hang free in a cavity which 

 is partly pollen chamber and partly archegonial chamber (fig. 147). 



When the body cell, or spermatogenous cell, is first formed, no 

 blepharoplast or centrosome has been demonstrated; but, as the 

 body cell enlarges and elongates, first one and then two very small 



