22.2 MAIXE AGRICULTURAL EXPERIMENT STATION. 



of tMs enzA^me is supported by the discovery of such a one, in a 

 species of Botr\-tis, b}^ ^larshall Ward.* 



It must not be understood, however, that the tube must always 

 experience difficulty in passing through the connective tissue, 

 since in some genera, especially in those of Liliacese, there is a 

 distinct central passage — ^the pollen-canal.f 



In the growing pollen-tube of the Angiosperms the large 

 vegetative nucleus first pushes out and takes its position near the 

 end of the tube. Later the smaller generative nucleus passes 

 by the former, after the growth of the tube is nearly completed, 

 and just before fertilization takes place, it divides into two. In 

 some instances a second division of one of these nuclei has been 

 obsenred, though as a rule only one of the nuclei is concerned in 

 fertilization. In some cases, however, as show^n by Strasburger 

 to be true of Monotropa Hypopitis.i an oosphere may be fertil- 

 ized by tw^o male nuclei. 



In general, the method of fertilization of Gymosperms is not 

 essentially different from that of Angiosperms, a fact first 

 pointed out b}^ Belajeff§ in the case of Tax us ha^cata and later 

 confirmed b}* Strasburger j | as the general rule. In other words, 

 the nucleus of the poUen tube is asexual, and fertilization occurs 

 b)- the luiion of one of the nuclei of the two cells, resulting from 

 the division in the cell-group within the pollen-grain. 



Previous to the work above cited, it had been supposed that 

 the nucleus of the pollen-tube was the male sexual nucleus, while 

 the cell-group in the pollen-grain was composed of asexual cells. 

 The misunderstanding concerning the nature of the cell-group 

 and the functions of the nuclei, arose from the erratic behavior 

 of the same in artificial cultures. In aU artificial cultures, the 

 cells composing the group were found to retain their position in 

 the poUen grain, thus being unable to reach the oosphere ; while 

 the nucleus of the gollen-tube being at the end of the tube, 

 would be in the natural position to fertilize the female nucleus. 

 Belajeff, however, in the work above cited, found that on the 



*Mar5haU Warrl: Ann. of Bot. II, 319, aSS?). 



IThls is particularly well shown in Tncca. SeeWebber, Am.Xat., XXVI, 774(1892) ; 

 also Eiley, Tnccas and their PoUination, III Bep. Mlssoari Bot. Gard., pp. 99-158. 



t Strasburger : Befmcht tmd ZeUth, Taf . IV, fig. 130; also referred to Bef . Lei den 

 Phan., 64. 



§ Belajeff: Bericbte der Dentsehen Iwt. GeseUsta, Bd. IX, 290, (1S91). 



Ill Strashm^er: Betmcbt bei den Gymiiospermen (1892). 



