DIV. II 



SPERMATOPHYTA 



5t>9 



same parts as the embryo in the Cycadeae, but the number of cotyle- 

 dons is frequently greater than two. 



(e) Gnetineae 



The last order of Gymnosperms, the Gnetineae, exhibit a peculiar and 

 isolated course of development. The microspores in their development and 



,.\\- 



FIG. 548. Apex of the embryo-sac of Gnetum Rumphianum shortly before the development 

 of the.female cells, u-k, Female nuclei ; mk, male nuclei ; PK, pollen-tube nucleus ; ps, pollen- 

 tube, (x 500 ) 



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

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

 in that two similar nuclei lie in the common protoplasmic investment. The 

 macrospores show more marked peculiarities. The macrospores of Ephedra and 

 Welwitschia have well-developed prothallia. Ephedra forms archegonia which on. 

 the whole resemble those of the Coniferae. Wdwitscliia has elongated cells with 

 2-5 nuclei which grow from the summit of the prothallus into the tissue of the 

 nucellus towards the entering pollen-tubes. Their significance as archegonia is 

 not clear. In Gnetum (Fig. 548) no prothallium is formed, but the embryo-sac 



