Derby — Crown Structure of Psaronius Braziliensis. 149 



Art. XIII. — Observations on the Crown Structure of Psa- 

 ronius Braziliensis ; by Oryille A. Derby. 



[Preliminary note in advance of the "Annaes do Servigo Geologico e 

 Mineralogico do Brasil" ] 



In the restorations of certain of the stem features of 

 Psaronius brasiliensis (this Journal, Nov. 1913) several vas- 

 cular strands are represented as rising from the plane of the 

 lowest cross section at hand, into the higher lying portions of 

 the stem hi the case of fig. 2, and into the air in that of fig. 3. 

 The latter figure suggests the idea that when in the living 

 plant the top of the stem stood at this level, all its vascular 

 strands here passed, in a similar manner, from the stem into 

 the members of the crown, standing free in the air, that they 

 supplied. In other words, the plane of this cross section was 

 then that of the junction of stem and crown, which though 

 not an absolutely level horizontal surface may be presumed to 

 have had its irregularities limited to so narrow a zone that, for all 

 practical purposes, this plane in the fossil may be considered 

 indifferently as that of the top of the stem or of the base of 

 the crown. It thus represents the plane, or narrow zone, that 

 was at one time occupied by the vegetative cone, or perhaps 

 more properly disc, of the growing plant. With continued 

 growth the vegetative disc would rise along the stalks of the 

 crown organs that protruded above it, partially burying them 

 in the rind-enclosed central parenchyma that, below it, was 

 being added to the stem. 



If this conception of the mode of growth of the plant be 

 admitted, a schematic restoration of the basal portion of the 

 crown of the living plant becomes possible. Any cross section 

 of the fossil stem may be regarded as representing the 

 plane of the vegetative disc at some moment in the life 

 of the plant and the portions of the strands that stand above 

 this plane can be plotted as rising free in the air. In life the 

 aerial portion of each strand must have had a parenchymons 

 covering which was modified on its outer surface into a more or 

 less tenuous dermal layer that only when sclerotized to an appre- 

 ciable extent would be preserved in a recognizable condition in 

 the fossil stem. Thus in an attempt at restoration, the repre- 

 sentation of the strands can be made with a fair degree of accu- 

 racy, provided that the successive cross sections of the fossil are 

 not too widely spaced, while that of the parenchymous covering 

 and of its relation to the fossil stem must be more or less 

 hypothetical. Such an attempt has been made in the accom- 

 panying figures (I, II, III, about two-thirds natural size), in 

 which Mr. Francisco Basto has skillfully plotted on three of 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII, No. 224.— August, 1914. 

 11 



