﻿TRUNK STRUCTURE. 



61 



Before discussing bundle types it should be noted that evidently, in studying 

 the leaf-base bundles of a trunk, it will always be necessary to cut at least three 

 serial sections from several adjacent leaf bases — preferably from a given portion of 

 the armor — in order to defi- 

 nitely trace the progressive 

 changes undergone by the 

 bundle pattern and in the 

 distribution of xylem and 

 phloem. One of these sec- 

 tions should lie near the cor- 

 tex ; the second should be 

 cut near the middle, and the 

 third from the peripheral 

 region of the armor. The 

 accompanying petiolar form 

 may of course be determined 

 by a section through the 

 crown of the trunk when 



i£>^,.r mm 









- .- SaflSMjsaas 



Fig. 30. — Cycadeoidea dacotensis. 



Showing bundle pattern of younger leaf bases and the heavy development 

 of centrifugal xylem. S. 506. 4. Fr. V, T. 214. 



young fronds are present, as 



is fortunately the case in scores of the trunks from the Black Hills. Further 

 serial sections woidd also be required to determine the exact order in which the 

 leaf trace splits up in the outer cortex and in the leaf-base insertion to form the 

 bundle pattern. Though such have not been prepared from any of the American 

 specimens, a certain degree of complexity may be expected, since Lignier has 

 shown that in C. micromyela (84, fig. 19) the single cortical leaf trace first tricho- 

 tomizes; then the median of these three main branches subtrichotomizes to form 



a n 



Fig. 31. — Cycadeoidea nigra, type. Boulder, Colorado. (A) Natural size ; (B) < 3. 



(A) Transverse section through the armor, cutting two adjacent leaf bases and their axillary peduncles at a dittance of several centimeters from the 

 cortex. The two lowermost bracts, or perchance pollen-bearing frond bases, of the upper peduncle are inserted a little beneath the level of 

 this section, while the lower peduncle is cut beneath the insertion of the lowermost of its lateral organs. The cylindrical bundle of the peduncle 

 and the bundle grouping of the leaf bases are both shown. (The remarkable trunk from which these sections were cut bears similar peduncles 

 to those here shown in the axil of every frond over large areas of the lateral surface, but no basal portions of strobili remain. ) 



(B) Transverse section cut distally parallel to preceding, showing five large bract or microsporophyll (>) bases surrounding central peduncle. 



