. Morphology of gymnosperms. Gymnosperms; Plant morphology. 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
. Morphology of gymnosperms. Gymnosperms; Plant morphology. 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
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