EMBRYO DEVELOPMENT AND POLYEMBRYONY 
Thuja (figs. 62-67) is a step higher, for here cleavage polyembryony 
does not occur normally. I find from my own studies* of this species that 
it occurs occasionally, probably coming from such embryos as shown in 
figures 64 and 66 by Land (21). In most cases, one of the embryo initials, 
Figs. 62-67. Stages in embryogeny of Thuja occidentalis. Figures 62-66, after Land 
(21), X 300. Fig. 87. Later stage showing apical cell of embryo, X 225, after Stras- 
burger (40). 
such as the lowest of figure 65, has a more favorable position and gives 
rise to the embryos, the others only elongating to form the suspensors. 
Whether only the terminal embryo initial functions or several of them, 
an apical cell stage is always found; a very conspicuous feature in Thuja 
(fig. 67). From my own studies* of Thuja occidentalis, I can also state that 
rosette embryos are found in a significant number of cases, but they are 
not found in Thuja orientalis. 
Tetraclinis probably represents a more advanced condition than Thuja, 
for it appears that only the terminal embryo initial functions. Saxton (36) 
reported no splitting embryos, but found that several tiers of the proembryo 
may contribute to the suspensor. However, the existence of cleavage 
polyembryony may easily have been overlooked. 
In Libocedrus, which was studied by Lawson (26), the embryogeny is 
also too little known to justify any conclusions, and the embryogeny of 
both Tetraclinis and Libocedrus should be reinvestigated to clarify some 
of these points. 
The Taxodineae and Cupressineae appear to be very similar on the 
basis of embryogeny, both groups showing an evolution from cleavage 
polyembryony to simple polyembryony, but our knowledge of both of 
these groups is at present very unsatisfactory. 
^ Unpublished work. 
