258 



MORPHOLOGY OF GYMNOSPERMS 



tetrad is not always linear, Coker having reported (85, 86) that 

 in Thuja, so far as he has observed, it is always tetrahedral. In case 

 there are several mother cells, they may all form tetrads. In Sequoia 

 sempervirens (92) the five or six mother cells form tetrads, and ten or 

 twelve megaspores begin to germinate, only two or three continuing 

 beyond the first division, and finally one becoming dominant (figs. 

 282, 283). The sterile megaspores in this species are quite prominent, 



282 



283 



Figs. 282, 283. — Sequoia sempervirens: fig. 282, longitudinal section of ovule, 

 showing six megaspore mother cells; pollen tubes are growing down between nucellus 

 and integument; March 12, 1902; X83; fig. 283, longitudinal section of nucellus, 

 showing eight germinating megaspores, each containing two nuclei; April 25, 1902; 

 X116. — After Lawson (92). 



being clustered about the upper third or fourth of the remarkably 

 elongated functional megaspore (42). In Cryptomeria japonica (93) 

 there are four tetrads formed, and the centrally placed megaspore of 

 the group of twelve to sixteen functions. In Libocedrus decurrens 

 (131) one to three tetrads appear, from which a single functioning 

 megaspore is selected; and in Pinus silvestris (363, 166) and Callitris 

 two embryo sacs have been observed. It may be inferred that two or 

 more mother cells and tetrads may occur in any of the Pinaceae, but 

 thus far they seem to be especially characteristic of the Taxodineae 

 and the Cupressineae. 



