SPERMATOPHYTES 



23s 



Sfii< 



antipodal region are multinucleate, and those of the micropylar region 

 become so (two to five-nucleate). Since there is no uninucleate cell in 

 the micropylar tissue, there is no archegonium initial, and hence no 

 archegonium. Instead, several of the multinucleate cells develop what 

 have been called prothallial tubes, which penetrate the overlying 

 nucellar tissue, and into them the nuclei pass, each nucleus being a 

 potential egg nucleus. It is at this point that the archegonium dis- 

 appears ; which seems to be associated with the fact that the egg 

 nuclei are differen- 

 tiated before wall 

 formation in the en- 

 dosperm has been 

 completed. 



Gnetum.- — In this 

 genus there is free 

 nuclear division as 

 before, but wall for- 

 mation does not oc- 

 cur, so that the em- 

 bryo sac at the time 

 of fertilization con- 

 tains only free nuclei 

 (fig- 531)1 and each 

 one of these free 

 nuclei is a potential 

 egg nucleus. This 



is the general angiosperm condition. Below the antipodal end of the 

 sac a remarkable nutritive (glandular) tissue is developed. 



Male gametophyte. — The male gametophyte of Gnetales is known 

 only in Ephedra. In its shedding condition the pollen grain of 

 Ephedra contains two persistent vegetative cells, and conspicuous 

 stalk, body, and tube nuclei (fig. 532). In this condition the pollen 

 grain rests on the exposed archegonium necks, and before the pollen 

 tube is formed the body nucleus divides into two equal male nuclei 



(fig- 533)- 



Fertilization. — The phenomena of fertilization vary with the struc- 

 ture of the female gametophyte. In Ephedra the pollen tube breaks 

 through the long and massive neck of the archegonium (fig. 529); in 

 Tumhoa it comes into contact with the prothallial tubes that are pene- 



532 



Figs. 532, 533. — Male gametophyte of Ephedra: 532, 

 pollen grain in shedding stage, containing two vegetative 

 nuclei (below), stalk {sin) and body (Jbn) nuclei, and tube 

 nucleus (above) ; 533, completed male gametophyte (after 

 beginning of pollen tube), showing in the tube the male cells 

 (m), stalk nucleus {s), and tube nucleus (/). — After Land. 



