﻿TRUNK STRUCTURE. 



59 



and show considerable variation in the same section. With regard to the specific 

 names of the sections figured it is to be added that these are simply those that have 

 been used thus far as a necessary, though arbitrary, convenience. A few supple- 

 mentary photographs of such sections are shown on plate xyii. 



^ K^fly 



*%&* " 



® "^ 





X T 9 8 



Tracing made from polished columnar surface of trunk, illustrating distorted symmetry of the old leaf base spirals 



due to fruit growth and emergence. 



Three of the originally symmetrical leaf base spirals running from left to right are numbered 1-11, I'-ll', 1"-11", — the right to left 

 order being 1 - 1 ", 2-2", 3-3", and so on. The smallest axis of fructification (b), is bisporangiate, and the three larger of the four dis- 

 tinctly ovulate cones present are so far advanced in growth that if all the cones borne by the trunk are of the same season, a monoecious 

 rather than a functionally bisporangiate condition would be indicated. The bundle patterns of the four leaf bases of the spiral above and 

 to the left are shown. (A layer of armor 1 to 2 cm. in depth was polished away to secure the surface traced. Cf. figure 26.) 



Leaf Base Bundle Grouping. — Figures 30-33. 



The general form and grouping of the leaf bases as thickly beset by and 

 packed in ramentum having been considered, it is next in order to take up struc- 

 ture ; and the most prominent feature in this connection is the bundle arrangement, 

 which can often be readily observed on the naturally weathered or eroded ends of 

 the leaf bases without the aid of thin sections. The illustrations of bundle group- 

 ing shown in the accompanying figures include all of the characteristic types 

 observed. In general the more or less concentric or horseshoe-like bundle, or pair 

 of bundles, which enters the leaf base first splits up into from ten to twenty mesarch 

 bundles, forming in transverse section a round to elliptical pattern with the xylem 

 directed inwards. But a depression soon forms on the superior side of this pattern 

 and gradually deepens, the final stage, as can be observed in the sections of the 

 petiole and rachis of young fronds, outlining a heavy V, with a tendency for the 

 ends of the arms to approach each other. (See figures 30 and 33.) 



There are, finally, so far as observed, two other types of bundle arrangement 

 of far more infrequent occurrence than that just described. In C. ingens {cf. fig. 

 33, No. 12) and, as is also evident from the figure of a portion of a leaf base, in 

 C. gigantea from the Portland Beds of the Isle of Purbeck (as given by Seward, 

 144, plate v, fig. 18) there is no definite bundle pattern, as the result most likely 



