SECT. II 



PHANEROGAMIA 



509 



The last order of Gyninosperms, tlie Gnetineae i^"), exhibit a peculiar and 

 isolated course of development. The microspores in their development and 

 germination show no essential differences from those of other Gymnosperms ; the 

 separation of the generative cells is, however, less clear and sometimes wanting. 

 Tlie macrospores show more mai'ked peculiarities. The macrospores of Ejihedra 

 and IVehvitschia have well-developed prothallia ; in the latter plant the archegonia 



FiQ. 471.— Pollen-grain of Lilinm Martagon and its germination, k, Vegetative nucleus of tlie 

 pollen-grain ; m, antheridial mother-cell ; (j, generative nuclei, (x 400.) 



are reduced to elongated cells penetrating the tissue of the nucellus. Since, how- 

 ever, more than one nucleus may be present in these cells, further investigation 

 into their function and significance is needed. In Gnetum, on the other hand, 

 either no prcthallium is formed but the embryo-sac becomes filled with protoplasm 

 in which art; numerous nuclei, or a protliallium is developed in the lower portion 

 of the embryo-sac while the upper portion contains protojilasm with free nuclei. 

 Each of the two generative cells from the pollen-tube fuses with a female nucleus. 

 Of all the fertilised cells resultii g from the penetration of a number of pollen- 

 tubes to the embryo-sac only one develops into an embryo (Fig. 470). 



