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RELATIONSHIPS. 



a closed group, such as .the existing and fossil cycads are here conceived to form. 

 Thus, for instance, stress is laid on the idea of the formation of a hypothetical mega- 

 spore supply bundle of sheathed concentric type in which the xylem and phloem 

 assume the simple form seen in fig. 129, 1. But we will further suppose this bundle 

 to be related to one that can give rise in the base of the seed to a radio-symmetric 

 and not a bilateral series of collateral strands, with much transfusion tissue. If at this 

 point, as might well be the condition at the base of a small inegaspore sustained 

 by a single bundle, a continuous zone of transfusion tissue were to branch off instead 

 of the small inverted collateral bundles as in Cycas, or if such a zone is in itself 

 really the more primitive cycadean condition, as is equally or even more supposable, 

 then we would have a bundle supply of the megaspore much like that of Cyca- 

 deoidea, derived by only slight changes involving the production of not a single 

 new element or course of change not already seen to be strictly cycadean. 



Fig. 132. — Poroxylon Edwardsii. Transverse section of the stem, showing two primary centripetal xylem strands and 

 adjacent tissues, for comparison with the two preceding figures and also with the leaf-base bundles shown in figure 34. 

 X 66. (From Scott, after Bertrand and Renault.) 



x. Primary centripetal xylem ; px, protoxylem ; x-, secondary xylem ; mr, medullary rays ; p, pith. 



Structure and significance of the axis of the cones. — The axis of the fruiting 

 cone is primarily a terminal growth from the main vegetative stem, persistent in 

 the ovulate strobilus of Cycas, fugacious in all the other forms. Plural axes appear 

 to result from dichotomy or polychotomy of the plerome, or else occur as lateral 

 branches from near the apex of the trunk. After dehiscence of the terminal cone 

 renewed plerome activity with sympodial stem growth takes place. The fugacious 

 axes retain a structure comparable in all larger features to that of the cyeadeoidean 

 cycads, in which, as described, the peduncle, while a branch of limited growth, is a 

 replica of the parent trunk, the collateral bundles of the woody cylinder of the 

 peduncles and cones having retained little or no centripetal xylem. In the cycads, 

 however, while the course of the sporophyll bundles through the cortex of the cones 

 is a direct and uncomplicated one, anatomical study has revealed the fact that the 



