35© GYMNOSPERMS 



embryo comes from a single egg by the splitting of the product of a 

 single fertilization, the term cleavage polycmbryony is used. Both 

 types are widely distributed in the conifers. 



In the Abietaceae, cleavage polycmbryony is a constant feature 

 of Pinus, Ccdrus, Pseudolarix, and Tsuga; while simple polycmbry- 

 ony is the rule in Larix, Picea, and Pscudotsuga. In Abies, cleavage 

 occurs in rare cases: simple polycmbryony is the rule. 



In the Taxodiaceae, cleavage polycmbryony is excessive in Scia- 

 dopitys, is not so frequent in l^axodium, and does not occur in Se- 

 quoia. 



In the Cupressaceae, cleavage polycmbryony occurs, at least oc- 

 casionally, in Juniperus, Thuja, Biota, Actinostrobus, Widdringtonia, 

 and Callitris, but not in Tetraclinis. 



In the Araucariaceae, neither Araucaria nor Agathis has any 

 cleavage polycmbryony. 



In the Podocarpaceae cleavage polycmbryony is not so prevalent. 

 A form of polycmbryony described as determinate cleavage poly- 

 cmbryony has been described for Dacrydium,^° and in Podocarpus 

 coriaceus some of Coker's figures suggest cleavage polycmbryony, 

 but most species of Podocarpus (for example, P. spicatus"''*), and also 

 Phyllocladus, Saxegothea,'*^'' and Microcachrys^^" seem to have simple 

 polycmbryony. 



In the Taxaceae Jager-'*'^ found that in Taxus baccata some of the 

 upper suspensor cells occasionally break away and form small em- 

 bryos; but even this last trace of polycmbryony is usually lacking, 

 and it docs not occur in the other genera. 



Simple polycmbryony occurs throughout the order except, of 

 course, in forms like Torreya, with only one archegonium. 



BucHHOLz'^^^^^ believes that cleavage polycmbryony is a primitive 

 character, and that simple polycmbryony has been derived from it. 

 In some forms, like the Araucariaceae, Ccphalotaxus, and some spe- 

 cies of Podocarpus, the cap, very probably not a primitive feature, 

 may prevent cleavage; and in others occasional cleavage is inter- 

 preted as a reversion. With this interpretation the Pinus type is 

 very primitive, and the Araucaria type, very advanced. 



The rosette cells, where they are present, are embryonic, and often 



