﻿1 66 REPRODUCTIVE STRUCTURES. 



As a rule, the best preserved bisporangiate axes, from the larger trunk types, 

 both columnar and branching, as seen in C. ingens and C. dacotensis, are, together 

 with the bracts subtending them, from 5 to 7 cm. in diameter, while the distance 

 from the summit to the origin of the peduncle traces in the xylein is from 10 to 

 15 cm. in the case of the larger trunks. With due care it has hence been possible 

 to cut radial longitudinal and transverse (tangential) thin sections, varying from 50 

 to 150 sq. cm. in area, and showing not only entire axes, but the manner in which 

 these are borne on the trunk as well, just as was done in the case of the more 

 advanced stages of the ovulate strobili. Measuring from these sections, the length 

 of the barrel-shaped peduncle is from 5 to 7 or more centimeters, and its median 

 vertical diameter about 2 cm., though frequently the peduncles are even larger 

 and much flattened. The length of the flower bud proper, as consisting of the 

 central ovulate strobilus and the surrounding innexed pollen-bearing fronds, is 

 from 5 to 7 and the diameter about 5 cm. The central ovulate strobilus is 

 usually about 5 cm. in length by from 1.5 to 2 cm. in diameter, and the fronds 

 bearing approximately mature pollen are from S to 10 or more centimeters in 

 length. But in Cycadella young strobili have been observed less than a centimeter 

 in diameter, or no larger than the minute silicified strobili of Cordaitcs from 

 Grand Croix. 



The form of the individual fronds of the disk has been greatly altered, con- 

 formably to space requirements in the uuexpanded condition. While preservation 

 is not so complete as in the case of young foliage leaves, the bundles of the fronds 

 are often indicated throughout their course by the preservation of the xylein 

 elements. Hence the disk bundle system may be clearly traced. This, too, has been 

 modified in conformity to the assumption of an annular insertion, with basal fusion, 

 into a disk of the originally discrete and spirally inserted or whorled once-pinnate 

 fronds. The pollen-bearing synangia are identically comparable in their sporangial 

 grouping and structure with the spore-bearing synangia of the existing tree ferns 

 of the genus Marattia. The pollen is considered to be five-celled, rather than four- 

 celled as in Ginkgo, or four to three celled as in existing cycads. 



Following the wilting down of the disk of microsporophylls the central 

 ovulate cones appear in most instances to have continued their growth ; whence 

 these strobili were functionally bisporangiate, and had so continued from the time 

 when ancestral Marattiaceoid ferns attained heterospory, although the most varied 

 forms and transition stages of moncecism and dicecism were doubtless exemplified 

 by the various members of the great plant complex to which the existing and fossil 

 cycads belong. 



As an immediate result of these anatomical studies it is therefore clear that 

 other and possibly much varied bisporangiate types may be expected in the older 

 strata. And as we have seen, elaboration of the structure of the present new types 

 of cycadean fructification has already accurately extended our knowledge of the 

 hitherto obscurely known Williamsonia. The known species of cycadeoidean 

 bisporangiate strobili now number upward of ten, distributed in four genera, indi- 



