﻿EXISTING AND FOSSIL CVCADS COMPARED. 



195 



borne free on the carpels of the less compactly set strobili of the genus Cycas, the 

 mature seeds of which, though not very greatly larger than the ovules, reach in the 

 species Cycas drcinalis the size of goose eggs. 



TRUNK STRUCTURE. 



The cycad stem consists 

 finally, as in the gymno- 

 sperms and dicotyls, in a zone 

 of wood, cambium, and bast, 

 inclosing a large medulla, 

 and enveloped by a strong 

 cortex ending in a layer of 

 periderm, giving rise to thin 

 bark. But there is this 

 great difference. The cortex- 

 supports primarily (see figs. 

 108, no, etc.) a primitive 

 outer investiture or armor 

 of old foliar bases, persistent 

 or not in different species 

 and with increasing age of 

 ,-• ,n Q c- 1 . , - tne plant, according to the 



rig. I (Jo. — fcxtremesot variation in size and torm ot trunks ol existing cycads. 



a 7 (i j r>r . if ti ■! rapidity of its excision, 



A. Zamia llondana DC X %■ Miami, I londa. L J ' 



a. Upper third of long and slender young trunk with leaves cut away, and surface tOgetlier Wltll tllC penplieral 

 lightly cleared of scaly and hairy material to show the slowly disappearing rem- . • f 4-1 «- •*. if 

 nana of old leaf bases. Further down, on lower two-thirds of the trunk, the pOl'tlOHS Ot tile COITCX ltSelt, 

 leaf bases are indistinct. Entire trunk subterranean, as well as that portion of 

 the perioles shown at summit- 



b. Transverse section of the same trunk cut farther down at the thickest point, 

 showing central pith or medulla, the xylem or wood zone, and the corneal 

 parenchyma irregularly traversed by vascular bundles. There is no armor, this 

 being replaced by a very thin corky layer. 



B. The same as the preceding, showing the unbranched subterranean trunk 

 with fronds cut away just above surface of ground. ■( ). 



(I) entire; (2) in longitudinal section: a. apex or punctum vegctarionis; b. cortical 

 bundles in early and less curved portion of their course; c, woody or xylem cylin- 

 der; d, medulla. 



C. Cycas circinalis. Basal portion of a columnar trunk about 8 feet high, 

 showing a heavy armor formed by the succession of foliage and carpel- 

 lary leaves. 

 The large scars are due to ihe excision of the petioles of the foliage, the smaller to 



that of the far more numerous carpellary leaves which actually form two-thirds of 

 the armor of old frond bases. From photograph, by Professor Evans, of a trunk in 

 CasUeton Gardens, Jamaica. 



by the upward and centrip- 

 etal advance of successively 

 forming layers of periderm. 

 The several tissue systems, 

 together with the action of 

 the periderm and the features 

 of the armor, may best be 

 described in sequence. 



The Medulla or Pith. 



The central pith usu- 

 ally occupies about a third 

 of the diameter of the stem, being more strongly developed than in any other 

 existing gymnosperms, and quite as prominently as in the Cordaitales and such 

 tree ferus as Cyathea medullar is. Various species of Zamia, Dion, and Cycas afford 

 commercial starch and sago in large quantities, whilst the pith of Enccplialartos 

 is eaten by the native Kaffirs, whence the common name of this plant, the 

 "Kaffir bread." 



In the early stages of growth the pithy ground-mass of cells filled with starch 

 grains is traversed only by gum canals, the only other structural modification being 

 the more or less well marked upward and outward lay of the somewhat larger cells 



