420 



GYMNOSPERMS 



enous cell, which is also the megaspore mother-cell, and a primary 

 wall cell, which builds up an extensive tissue above the mother-cell. 

 Often, there may be more than a single sporogenous cell."' 



The megaspore mother-cell produces four megaspores, of which 

 more than one may germinate.'"' Coulter found that twelve and 



twenty-four are the x and 2x num- 

 st bers of chromosomes in Gnetum 

 -0 gnemon."*^ 



THE MALE GAMETOPHYTE 



The microspore is the first cell 

 of the male gametophyte. The 

 microspore of Gnetum gnemon, up 

 to the shedding stage, is shown in 

 A, B, and C of fig. 392. In this 

 series, by Thompson ,^^° there does 

 not seem to be any prothallial cell, 

 the first division giving rise to a 

 tube nucleus and a generative 

 nucleus, the latter soon dividing to 

 form a stalk nucleus and a body 

 nucleus. No cell walls are either 

 figured or described ; but in Gnetum 

 sp., in Z) and E of the same figure, 

 a body cell is organized. 



In the complete elimination 

 of even a single prothallial cell, 

 Gnetum has reached the angio- 

 sperm condition, where there are 



no prothallial cells, except in extremely rare cases, where they are 



regarded as reversions. 



At the first division of the microspore of G. africanum, Pearson 



found a very delicate wall, which soon disappeared, and no wall was 



formed at the second division. 



It is easy to follow the growth of the pollen tube, because the 



pollen grains frequently germinate while still in the micropylar 



tube. The exine is cast ofl, and the intine grows into a tube, begin- 



FiG. 392. — Gneltim: male gameto- 

 phyte: A, young microspore: B, a little 

 older; /, tube nucleus; g, generative cell: 

 C, t, tube nucleus; b, body cell; st, stalk 

 nucleus; D, end of pollen tube; st, stalk 

 nucleus; b, body cell; /, tube nucleus: E, 

 end of pollen tube, later stage; /, tube 

 nucleus; m, sperms. A-C, Gnetum gne- 

 mon, X 1,200; D, E, G, sp.; D, Xsso; 

 E, X 800.— After Thompson.'^" 



