﻿EXISTING AND FOSSIL CVCADS COMPARED. 



205 



The Armor. (Figs. 15-28, and 101, 108, ill, 113, etc.) 



The outer investiture or armor of the cycadean trunk consists in the pali- 

 saded and spirally arranged old foliar bases seated on the cortex and left behind as 

 the successive crowns of foliage and fructification wilt down, together with such 

 hairy material as these may bear. Beneath the 

 wilted ends of the foliar bases a periderm arises 

 in the still living tissue and from it cork and 

 bark cells are formed as the outer and protecting 

 surface of the armor. But as the formation of 

 cork may go on indefinitely the armor conies to 

 vary greatly in its amount and persistence, as 

 explained below in connection with the descrip- 

 tion of the periderm. The armor, composed of 

 the system of leaf-base and carpellary leaf-base 

 spirals, or occasional interspersed peduncles, is the 

 most characteristic outer feature of the cycadean 

 stem, the one best known and first remarked 

 upon. It is in every respect of ancient and fern- 

 like character, and its variable development in the 

 different species and strong tendency to disappear 

 is probably increasing with the age of the group. 

 At least with increase in height and age of the 

 trunk the armor disappears from below, and is 

 hence in any given trunk always thickest near 

 the summit. It may be three or more inches in 

 thickness in large trunks of Cycas, producing the 

 very characteristic summits to be noted in various 

 accompanying figures. The low-growing trunks 

 of Encephcdartos are also heavily armored. In 

 some of the forms of Cycas and Macrozamia the 

 armor is little persistent, and in Zamia and Stangeria it is but very lightly devel- 

 oped for only a short distance beneath the leafy crown, the naked or "tuberous" 

 trunk being covered by the periderm with its cork and bark after the manner 

 next to be described. 



The Periderm. 



The periderm is structurally made up of two layers — a thin outer layer of 

 crushed cork giving rise to bark and a thicker inner layer of phelloderm. It is a 

 centripetally advancing and excising tissue. Below whatever armor is present, it 

 forms with its cork and bark the final outer covering of the trunk. Its history in 

 the cycads is this : As soon as a frond wilts down a transverse peridermal layer 

 forms in the outer living portion of the base left behind ; and this is succeeded 

 by another and another until all of the base is cut away to the cortex, the periph- 

 eral portions of which are next similarly excised with the formation below of a 



Fig. I 16. — Cycas siamensis (?). X Y%. 



This nearly armorless bulbous trunk affords a strong 

 contrast to other heavily armored columnar trunks of 

 the genus Cycas. and is in outer appearance and form 

 closely comparable to the Sandwich Island palm 

 Pritchardia Martii. It is also much like the subterra- 

 nean Zamias. 



