2IO 



GYMNOSPERMS 



The mitosis for the formation of the ventral canal cell and the egg 

 takes place in the second week in September. A definite cell wall is 



formed. Consequently, the de- 

 velopment is quite different from 

 that in the cycads, for in none of 

 them is a wall formed at this mi- 

 tosis, the two nuclei lying free in 

 a common mass of protoplasm. 

 As far as this feature is concerned, 

 Ginkgo is more primitive than 

 the cycads. 



In the fmal stages of the devel- 

 opment of the female gameto- 

 phyte Ginkgo differs from the cy- 

 cads. Instead of the cup-shaped 

 archegonial chamber of the cy- 



Tnt 



Fig. 227. — Ginkgo biloha: upper part 

 of female gametophyte, showing two 

 archegonia and the crevice-like arche- 

 gonial chamber, also the tent-pole pro- 

 longation of the gametophyte supporting 

 the nucellus. The swollen ends of two 

 pollen tubes are shown, just ready to 

 discharge the sperms (September 9); 

 X 24.— After HiRASE.«3 



CLP 



Fig. 228. — Cycadinocar pus angustodunensis: upper part of ovule, showing striking 

 resemblance to Ginkgo; mi, micropyle; inl, integument; ar, archegonia; pc, pollen cham- 

 ber with pollen grains; nii, nucellus; pr, female gametophyte. — After Renault.-'^s 



cads, Ginkgo has a circular crevice, surrounding a mass of solid tissue 

 upon which the nucellus rests, like a tent on a pole (fig. 227). 



In general topography, Ginkgo, at this stage, bears a striking re- 

 semblance to Cycadinocar pus angustodunensis, a paleozoic seed 

 assigned to the Cordaitales (fig. 228). 



