﻿TRUNK STRUCTURE. 



55 



have doubtless retained their free growth of ramentum ever since the Paleozoic, it 

 is scarcely sufficient to suggest the action of the simpler ecological factors alone ; 

 aud it is not only the ramentum that has failed to continue in most gymnosperms 

 and in the augiosperms, but the entire armor — bark formation taking place rapidly 

 and evenly over the entire stem, so as soon to excise not only all vestiges of 

 ramentum, but the old leaf bases and scars as well. 



Fig. 24. — Cycadeoidea sp. T. 755. 



Tangential section through armor about 4 cm. exterior to the cortex. Leaf bases in but 

 slightly distorted spiral order surrounding an isolated peduncle and its bracts. Peduncle 

 and leaf base bundle patterns indicated. Natural size. 



LEAF BASES. 

 Outer Features and Arrangement. — Figures 19-29. 



Persistent leaf bases are especially characteristic of cycads and tree ferns. As 

 the fronds of the leafy cycad crowns wilt down, layers of periderm arise in the 

 bases some distance out from the cortex and give rise to thin protecting bark, 

 which arrests further wilting after petiolar excision. The continuous enveloping 

 "armor" of spirally arranged old leaf bases thus formed is the most striking outer 

 feature of the cycadean trunk, although often slowly excised from about the base 

 by the further formation of successive layers of periderm, with formation of the 

 variant types of figure 15, showing a heavy and a light armored cycad. This pro- 

 gressive armor elision is more characteristic of existing than of extinct forms, a fact 

 which well accords with the much more profuse ramentum and rather heavier armor 



