404 



GYMNOSPERMS 



been modified by extensive nuclear fusions. We see no reason for 

 adopting the term "trophophyte," which has been proposed. 



Fertilization. — The pollen tubes growing down into the nucellus 

 and the prothallial tubes growing up into it meet, and fertilization 

 takes place (fig. 378). A pollen tube and a prothallial tube come into 

 contact, the tube walls at the point of contact become dissolved, and 



/cr 



Fig. 379. — WelwHschia mirabilis: early embryogeny: A, first division of zygote; ps, 

 primary suspensor cell and, below it, the cell from which the embr>'o is to develop; B, 

 later stage, with inner cortical ring (icr) and terminal initial cells (/c) ; C, still later stage; 

 ps, primary suspensor; icr, cells of inner cortical ring; ocr, cells of outer cortical ring; the 

 embryonic plate (e), ring (.v), and cap (c), are each represented in section by two cells; 

 A, X700; B and C, X30S. — After Pearson. ■"'' 



the nucleus of the prothallial tube passes into the pollen tube, where 

 it fuses with one of the sperm nuclei. Pearson found no independ- 

 ence of the chromatin of the gametes, as it had already been de- 

 scribed for Pinus and Juniperus, but found a complete fusion, with 

 a resting nucleus, before any division of the zygote. Nearly all of 

 the cytoplasm is contributed by the pollen tube. The zygote nucleus 

 passes from the pollen tube into the prothallial tube on its way down 

 to the endosperm. 



Embryo. Tho zygote elongates and its nucleus divides. As in 

 Ephedra, the wall begins to form at the periphery and closes in to- 



