ig2o] BUCHHOLZ— POLYEMBRYONY 163 



Another point in favor of the view that cleavage polyembryony 

 is a primitive feature is the fact that Pinus is known to be very 

 old historically. This genus has come to be regarded by paleo- 

 botanists as one of the very oldest conifers (6). On the other 

 hand, Jeffrey (9, 10) has reached this same conclusion on the 

 basis of anatomy. 



An additional argument that cleavage polyembryony is primitive 

 comes from a consideration of the relation that the pine embryo holds 

 to the known steps in the embryo development of other conifers. 

 There are several lines of evolution which have arisen from a primi- 

 tive type of embryo like Pinus. One of these is the abietineous 

 evolution shown in this investigation, the series beginning with 

 Pinus and culminating in Pseudotsuga. Another evolutionary 

 series begins with Pinus, involves some of the Cupressineae and 

 Taxodineae, and culminates in Gnetales, a line in which cleavage 

 polyembryony has been retained. Ephedra has a modified form of 

 cleavage polyembryony, w r hich associates it with Coniferales on 

 the basis of its embryogeny. Other evolutionary lines may have 

 been derived from the Pinus type of embryo, as described else- 

 where (3). This is therefore another strong argument that the 

 pine type of embryo is very primitive. 



Strasburger (18) has reported that Picea develops only one 

 embryo per archegonium, and his results are thus verified by this 

 study, but he did not attach any significance to the question of 

 whether or not a separation of the embryos occurs. Other investi- 

 gators in dealing with the embryos of the Abietineae have likewise 

 failed to make this point clear, and the embryogenies of some genera, 

 such as Cedrus, Tsaga, Abies, and Larix, have been partially investi- 

 gated in proembryo stages only. 



The proembryo of Pinus has been most extensively studied, 

 described, and figured by Chamberlain (4), Coulter and Cham- 

 berlain (5), Miss Ferguson (7), and Miss Kildahl (ii), each 

 investigator adding a few additional. stages and details. The facts 

 brought out by these investigators are in harmony with the inter- 

 pretation given to the proembryo in this paper. 



The embryogeny of conifers has not usually been undertaken by 

 morphologists as a distinct problem, but the stages described and 



