I902J A MORPHOLOGICAL STUDY OF THUJA 257 



walls are formed. In Podocarpus sixteen or thirty-two free 

 nuclei are formed; and in Taxus and Cephalotaxus a like number. 

 In Thuja the number of free nuclei is constantly eight, while In 

 Pinus and its allies the number is only four. Even so small a 

 number as two is reported by Strasburger (11) as occasionally 

 occurring in Ephedra; and in Gnetum, according to Lotsy (10). 

 there is no free nuclear division at all. It would seem that 

 there is a gradual transition from an extensive free nuclear division 

 to a condition in which the first division of the c^^ is followed 

 immediately by the formation of a wall. In the development of 

 the embryo Thuja has not advanced so far as forms like Pinus, 

 although in its cyclic arrangement and in the reduction of the 

 male gametophyte it has proceeded much further, 



A wall transverse to the long axis of the oospore now sepa- 

 rates the eight nuclei into two groupsj each group having four 

 nuclei. The nuclei of the lower group are completely walled in, 

 and from these the embryo develops. Walls parallel to the 

 long axis of the oospore extend from the transverse wall upward 

 a short distance into the cytoplasm of the oospore, thus walling 

 in the nuclei on three sides, but leaving them freely exposed to 

 the cytoplasm above {Jig. 24). The ventral nucleus is still 

 ( visible but shows marked signs of disintegration. 



' The four nuclei of the upper group now divide simultaneously, 

 and w^ls are formed transverse to the long axis of the arche- 

 gonium {figs. 24-23). The proembryo at this time consists 

 of twelve cells; two tiers of four cells each, and the group of 

 four in the tip, which were formed at the time shown in Jig. 2j, 

 The upper tier of incompletely walled nuclei, produced by the 

 division shown in Jig. 24, soon disintegrate. The second tier 

 rapidly eloncrates and forms the four suspensors {Jig. 26). 



r n 



A 



i 





r A single embryo is developed from the lowest group of cells, 



which is thrust downward by the suspensors. In the Abieteae 

 each cell forms an embryo. Coulter (6) has shown that in 

 Pinus Laficio two embr^-os maj- develop at the end of a single 

 suspensor. In Thuja there is a tendency to form two groups of 



I cells. The line of division is clearly apparent in the earlier 



f 



