﻿102 VEGETATIVE FEATURES. 



parts to Boivenia and has some features near to Zamia vernicosa, a very doubtful 

 species known only in cultivation. As in Cycadeoidea ingens, the parallel in struc- 

 ture to the living cycads is thus once more seen to be so close as to recall generic or 

 even specific likenesses of leaf structure. 



REMARKS ON GENERAL CHARACTERS OF CYCADEOIDEAN FOLIAGE. 



Our knowledge of isolated fossil imprints of mature cycad fronds is now sup- 

 plemented with remarkable completeness by the fortunate discovery of young 

 adventitious fronds and leafy crowns in various stages of growth, as borne by a 

 number of silicified trunks from the Black and Freezeout Hills. Indeed, with all 

 the collateral evidence at hand, thus including not only the mature imprints — which 

 in the course of time can no doubt be referred with accuracy to the genera and 

 species of silicified trunks — but exact details of prefoliation and frond structure as 

 well, the picture of Cycadeoidean foliage becomes unexpectedly complete. Especially 

 is this so when it is recalled how abundant the imprints of cycadaceous forms are, 

 and yet how rarely both trunks and leaves occur associated as in WUliamsonia, 

 while unquestioned organic connection of foliage has been seldom determinable 

 hitherto. In general it may be said that in the light of this completer knowledge 

 it would in many instances be well-nigh impossible to separate isolated fronds of 

 the Cycadeoidese from any forms like those of the Cycadacese unless perchance the 

 structure of the rachis were indicated in both. Moreover, it becomes a reasonable 

 conclusion from the closeness of this parallelism that it is far more probable that 

 the main features of leaf structure in the fossil and existing cycads were developed 

 in a common ancestry than that there has been independent homoplastic develop- 

 ment of similar foliar structure of more or less xerophyllous type in two long- 

 separated groups of cycads. 



It remains, however, to draw some inferences as to the seasons to which the 

 above-described foliar growths belonged, and as to the general habits of trunk 

 growth. In all the specimens lateral organs of all kinds, whether flowers, bracts 

 branches, or leaves, were preceded on the surface by a dense surrounding and 

 enveloping growth of ramentum. Indeed it is largely to this peculiarity that we 

 owe the silicification of these various organs. Had any of the organs been excep- 

 tions to this habit of growth it is doubtful if a record of such would have been left. 



Owing to this profuse initial growth of ramentum it often happens that what 

 at first sight appear to be terminal helicoids of leaf bases are found, on making a 

 thin section, to consist simply in masses of ramentum, the individual scales of which 

 are five or more centimeters in length and oriented to the successive young leaf 

 axes hidden beneath. Such a section through a profuse terminal growth of 

 ramentum is shown in photograph 5, plate xix. This large and handsome trans- 

 verse section was cut 5 cm. beneath the summit of a branch referred to Cycadeoidea 

 Marshiana. The tips of several fronds have failed of preservation, but, with this 

 exception, the entire mass is seen to be composed of the transversely cut scales 

 arranged in helicoidally disposed areas. In such cases only a large, deeply cut sec- 

 tion of the terminal bud will disclose the organs thus deeply embedded and 

 hidden in a luxuriant growth of young ramentum such as is only fully paralleled 



