348 MORPHOLOGY OF GYMNOSPERMS 



feature. Among the Taxaceae, however, there is wall-formation 

 in connection with the mitosis which produces the four-nucleate stage 

 (Torreya), the sixteen-nucleate stage (PhyUocladus and sometimes 

 Cephalotaxus), and the thirty-two nucleate stage {Taxus, Cephalo- 

 taxus, and Podocarpus). A large number of free nuclei before wall- 

 formation is a feature of the proembryos of Cycadales and Ginkgoales, 

 and therefore it has been regarded as a primitive character. In 

 general, this is probably true, but that it cannot be applied within a 

 range of one or two divisions is evident from the fact that the extreme 

 limits of free nuclei among conifers (two and sixteen) are found in 

 Torreya and Taxus, two of the most closely allied genera. 



THE SUSPENSOR AND EMBRYO 



Suspensor-formation as observed in Torreya taxi folia (loi) is 

 probably true in general of all the genera of Taxaceae (fig. 403). 

 Among Pinaceae the elongation characteristic of suspensor-formation 

 so often appears to be definitely restricted to a single tier of proembry- 

 onic cells that it has produced the impression that the suspensor is 

 necessarily an organ of definite origin. The elongation may involve 

 every tier of the proembryo, and the suspensor is the total product of 

 this elongation. In Torreya elongation begins in the uppermost 

 tier ("rosette") of the proembryo, which is probably always the case 

 when this tier is closed, and extends gradually downward, tier after 

 tier, until it includes the proximal region of the meristematic cylinder 

 produced by the embryo-forming cell. In this case, therefore, every 

 tier of the proembryo "contributes" to the suspensor. The same 

 phenomenon was observed in Thuja (72), among the Pinaceae. 



Usually a single embryo is developed at the end of the suspensor, 

 but in Podocarpus the constituent cells of the suspensor have been 

 observed (69) to separate, resulting in the formation of several 

 embryos from a single egg. Secondary embryos have been observed 

 in Cephalotaxus (130) also to bud from the group of embryo-forming 

 cells; and in Torreya such embryos have been seen (loi) to bud 

 from the suspensor region during the second season. In Cephalotaxus 

 (124, 130) a series of long haustorial "embryonal tubes" develop 

 early from the proximal cells of the embryo. 



The single embryo-forming cell of Torreya (loi) develops a cylin- 



