﻿53 



VEGETATIVE FEATURES. 



Two further differences, due rather to abnormal features of preservation than 

 peculiarities of growth, require mention. Owing to the frequent failure of the ends 

 of the leaf bases to silicify, they are often represented by cavities more or less 

 encroached upon by the surrounding ramentum before complete silicification took 

 place. Also, in some trunks in particular, there is a much greater ridging and 

 furrowing of the leaf bases than appears to be normally present. This would seem 

 to be due to some kind of contraction during or else to desiccation previous to 

 the process of silici- 

 fication. In figure 

 19 an example of a 

 normal leaf base of 

 Cycadeoidea ingots is 

 shown. But this and 

 the closely related C. 

 Jenneyana frequently 

 present the very 

 greatest variations in 

 transverse sections, 

 as is well illustrated 

 in figure 27. As there 

 shown, a few normal 

 transverse sections of 

 leaf bases are present, 

 but most of the bases 

 have undergone very 

 pronounced changes 

 in size and form, so 

 that taken by them- 

 selves it would be 

 impossible to deter- 

 mine the true form or 

 species. When now 

 the varying height at 

 which leaf bases may 



be preserved in different species from widely separated localities is further considered, 

 together with variations in size and age of individual plants, it will be fully clear that 

 size and regularity of leaf-base spirals are most inconstant. Subject to such endless 

 variation, leaf base features must hence only be used as specific indices with the 

 greatest care and circumspection, supplemented by the study of thin sections. As 

 macroscopic leaf base characters have been so much used in establishing species in 

 both Europe and America it has been deemed of importance to insert the series 

 of drawings, figures 18-29, showing how subject the armor sections of fossil cycads 

 are to variation and how little they may in themselves be relied on in establishing 

 species. As will be noted, in many cases the leaf-base bundle patterns are indicated 



Fig. 27. — Cycadeoidea ingens. T. 568. 



Transverse section of armor of a trunk about 40 cm. in diameter, cutting several young 

 fruits and showing much irregularity in the form of the leaf bases. Tangential to 

 cortex at a distance of 5.7 to 6 centimeters. X \- 



