220 MAINE AGRICULTURAL EXPERIMEXT 5TATI0X. 



In the Angiosperms the two nuclei are free in the general cell 

 of the grain, but in Gymnosperms there is a different condition. 

 In the latter as shown by Strasburger,* the pollen-grain, while 

 still in the anther, e. g. in Larix Europcea, or after it reaches the 

 nucleus, as in Taxus, divides into several distinct cells. The 

 steps in this division are as follows : The undivided pollen- 

 grain first separates into a large and a smaller biconvex cell, the 

 latter being crowded against the side of the wall of the pollen- 

 grain. Soon another cell is cut off from the large one, and 

 pressed closely against the first, both being much flattened, then 

 a third much more strongly arched cell is cut off from the large 

 one. This third is placed over the other two, but instead of 

 flattening out, divides later into a small stalk cell {Stielselle) 

 and a larger body ceU. 



As a result of the above mentioned divisions of the pollen- 

 grains of Gymnosperms, we find then, at the time of pollina- 

 tion, three cells ; the large cell which forms the poUen-tube, the 

 smaU stalk-cell, and the body cell, which later by division gives- 

 rise to two cells containing the male nucleus. 



On the Germination of Pollen and the Grozvth of the Pollen- 

 Tube: The first step of the coming together of the male and 

 the female elements is that of pollination, the conveyance of 

 pollen from the anther to the stigma. The time inters-ening 

 between this transfer, and the actual process of fertilization, 

 may var}* from a few hours in some species, to many months in 

 others. According to Schleiden, Cereus grandiHorus, with a 

 style six inches long, requires but a few hours; Colchicum 

 autumnale, with a stjde 13 inches long, 12 hours; Pinus Sylves- 

 fris, almost as many months. 



The fact that the pollen grain after falling upon the stigma, 

 goes through a process of germination, in its gross character- 

 istics comparable to that of a seed, has long been known. In 

 1871 this germination was carefully studied and demonstrated 

 by A'an Tieghem, j who compared the phenomenon to the germ- 

 ination of the spores of lycopods and many ferns, the pollen- 

 tube corresponding to the .prothallium of the latter. 



* strasburger. Befmctit bei den Phanerogamen, -2. 



^ Van Tieghem: " Rechercbes pbysiologiqne sar la vegetation dn pollen," etc. 

 Ann. des Sci. Xat., (Bot.) be Ser., Vol. 12, p. 31-2. (1S71.) 



