﻿TRUNK STRUCTURE. 6j 



cycads in their simpler cortex, all the other main features of structure and arrange- 

 ment presenting a striking agreement. 



Course of leaf trace through cortex. — The usually direct course of the cycade- 

 oidean leaf traces was first described by Carruthers (24). In his description of the 

 new genus Bennettites he says: 



' ' The vascular tissue for each leaf springs from the woody cylinder in a single 

 large compact bundle [(/'. the present text-figure 35] , which as it passes outward breaks 

 up into the different bundles required for the service of the leaf." 



Concerning the lateral fructifications it is added: 



" The axils of a large number of the petioles bore short branches. These axillary 

 organs are important features in this group of fossil plants. In some fragmentary 

 specimens every axil is occupied by a bud, as described by Mr. Brown ; but more 

 frequently the majority of leaves are without them. . . . These organs, however, 

 are not properly buds, for although they do not appear to have pushed themselves 

 beyond the permanent bases of the leaves, they are fully developed organs, and differ 

 from the secondary axes of Mantellia, which are generally broken off beyond the surface 

 of the permanent bases of the petioles, and show there a woody cylinder agreeing in 

 structure with the principal axis of the plant." 



These are interesting observations. A much more extended description was, 

 however, later given in the important restudy by Sohns-Laubach (156). He com- 

 ments on the general resemblances of the woody cylinder to tree ferns, and while 

 in the main showing the correctness of Carruthers' description, suggests the likeli- 

 hood of the presence of a more complicated arrangement as follows : 



" The lateral fruit-bearing shoots of Bennettites are, according to Carruthers, of 

 axillary origin, as previously noted by R. Brown. They are by no means present on 

 every leaf axis, but on the contrary in most cases absent.* 



" I may also remark that while this conception is most probable and certainly near 

 the fact, the absolute conditions are well-nigh impossible to determine, though it is 

 but a short time since they were wholly understood even in the living cycads. So far 

 as I can determine from the sections at hand the position of the axes of fructification 

 is wholly irregular. In places they emerge singly from the armor, and at other points 

 they are in actual contact in thick-set groups. In the latter case one can not escape 

 the impression that there may be lateral branching of the more advanced axis. Were 

 this the fact, bushy systems of closely set axes could be formed with occasional isolated 

 axes passing out between the leaf bases of the armor. An absolute proof of such a 

 condition could not be readily made, even if unlimited material were at hand for the 

 cutting of serial sections. The leaf bases lying between the fruits are for the greater 

 part much deformed into variously irregular transverse sections, and the identification 

 of both kinds of organs side by side is usually impossible. Where the fructification axes 

 occur singly, there is always an exactly underlying leaf base, which is in other cases 

 thrust entirely aside. But the pressure of the closely set organs of the armor upon 

 each other must have obliterated most of the original contours. While, however, the 

 material present does not permit an absolute determination of the axillary branching, 

 there is absolutely nothing to indicate that the entire stem, as in the living cycads, is a 

 sympodium. For there has not been found in the case of any Bennettites sections at 

 hand a single trace of the characteristic bundle systems which enter the medulla of 

 living cycads and indicate their apical fruiting." 



*This is, of course, true of Bennettites Gibsonianus and the more closely related species, but not 

 of such forms as Cycadeoidea >iii, r, ' a (<"/ • figure 31). and notably C. Stantoni from the California Cretaceous 



