SIM-.RM ATOPIIVTES 



227 



The cotyledons are not steadily two in number, as in the preceding 

 groups of gymnosperms, but vary from two (mostly so in Cupressineae 

 and Araucarineae) to as many as fifteen (in the Abietineae). It is be- 

 cause the pines and their allies are the most familiar gymnosperms that 

 gymnosperms are commonly described as polycotyledonous ; but it 

 should be remembered that the occurrence of more 

 than two cotyledons is a feature of only two tribes 

 of Coniferales (Abietineae and Taxodineae), and 

 that two cotyledons occur in some of the members 

 even of these tribes. 



Conclusions. — It seems most reasonable to 

 conclude that the Coniferales have been derived 

 from the paleozoic Cordaitales, which also gave 

 rise to the Ginkgoales. The Coniferales, however, 

 have retained fewer primitive characters than the 

 Ginkgoales, and are especially noteworthy in 

 having lost the swimming sperms. 



In comparing the six tribes of Coniferales, the 

 testimony as to their relationships is very con- 

 fusing. The testimony obtained from the geo- 

 logical record is necessarily incomplete, but so far 

 as it is available the relative ages of the tribes are 

 as follows: The Abietineae have been traced to 

 the Paleozoic, and in all probability are the oldest 

 of the Coniferales. The Araucarineae have been 

 traced through the Mesozoic, in which period they 

 were very abundant ; and in all probability they 

 are but little younger than the Abietineae. The 

 Taxodineae and Cupressineae are recognizable in 

 the Lower Mesozoic; while the Taxaceae (Podo- 

 carpineae and Taxineae) are not known below the 

 middle Mesozoic. It may be that this sequence 

 indicates the actual sequence of the tribes, but it is 

 hard to reconcile it with the morphological characters detailed above. 

 In any event, the fact that Coniferales as a whole have developed from 

 the Cordaitales seems to be sufficiently clear, and is all that need con- 

 cern the elementary student of the group. It should be remembered that 

 origin from Cordaitales means also a connection through them with the 

 ferns, and therefore that all gymnosperms have descended from ferns. 



Fig. 5 10. — Embryo 

 of Pinus, showing 

 (above) two cells of the 

 rosette, two cells of the 

 suspensor, and an early 

 stage of the terminal 

 embryo. — After Coul- 

 ter and Chamberlain. 



