300 



MORPHOLOGY OF GYMNOSPERMS 



the origin by splitting becomes difficult to explain, and it is believed 

 that in some cases the number may be increased by the coming in of 

 extra primordia, probably displaced from the first stem node to the 

 cotyledonary node. The occurrence of more or less complete cotyle- 

 donary tubes was also found to be quite general, twenty species being 

 cited. 



On the other hand, it must be remem- 

 bered that these same facts may be used 

 also as an evidence that the dicotyledonous 

 condition has arisen from the fusion of 

 more numerous cotyledons. It is evident 

 that the question is at present an open one, 

 but it must be remembered that probably 

 our oldest group of Coniferales, older even 

 than the Cycadales and Bennettitales with 

 which we are acquainted, is the extreme 

 illustration of polycotyledony, while the 

 youngest of the Coniferales are dicoty- 

 ledonous or nearly so. 



It has long been expected that partheno- 

 genesis would be found to occur among the 

 Pinaceae, and recently Saxton has reported 

 (158) that Pinus Pinaster ("cluster pine"), 

 growing at Cape Town, is parthenogenetic, 

 the nucleus of the egg dividing, and the 

 divisions continuing to all stages of the 

 prbembryo, when there was no trace of 

 pollen tubes in the nucellar cap, or tubes 

 containing no nuclei were only part way 

 through the nucellus. The series was 

 close enough, and the comparison with 

 stages in the normal embryo complete enough, to make the con- 

 clusion reasonably safe. 



Haydon (126) has made an interesting comparison between the 

 theoretical and actual output of an ovulate strobilus of Pinus silves- 

 tris. It seems that such a strobilus might produce theoretically about 

 1,500 proembryos, which means that number of successfully fertilized 



Fig. 371. — Pinus Laricio: 

 diagram of longitudinal sec- 

 tion of seed; the three layers 

 of the seed coat are not sepa- 

 rated from each other as 

 shown in the diagram, but 

 are tightly appressed, and the 

 outer maybe quite incomplete ; 

 n, remains of nucellus; i, re- 

 mains of inner fleshy layer; 

 s, hard layer; 0, remains of 

 the outer fleshy layer. 



