﻿mSPORANGIATE AXES. 



157 



HISTOLOGICAL DETAILS. 



The disk. — The ground tissue of the disk is not well preserved, but its bundle 

 system, as in the case of the central cone, is well marked by very similar xylem 

 groups. In transverse sections these are seen to be composed of from 25 to 40 or 

 50 cells irregularly arranged, as shown in figure 80 c. In the longitudinal section 

 spiral tracheids predominate, other cell forms of the wood being difficult to detect, 

 mainly owing to the small size of the bundles. The grouping of these bundles as 



Fig, 81. — Cycadeoidea dacotensis Macbride. 



33. 



Portion of a transverse section through unexpanded microsporophylls of a bisporangiate strobilus. cutting a rachis and adjacent synangia. In the 

 central synangium the outer covering of heavy-walled prismatic cells is seen to be followed by a thin-walled layer from one to several cells thick. 

 to which adhere the sporangial loculi closely ranged in two rows — one on each side of the synangium. Each loculus is usually delimited by a 

 thin band of collapsed cells with adherent pollen grains, and each row of loculi is bounded on the inner side by well-defined tissue a single cell 

 in thickness, except between the angles of adjacent loculi, where there is a thickness of several cells. Thus are formed the two opposed inner walls 

 of the two halves into which the synangium splits in dehiscing. Cut more obliquely the two inner walls present a striate appearance. The tips 

 of the three synangia on the upper side of the hgure are cut very obliquely, and comparison with the other synangia hence shows the position of 

 the dehiscence slit in the outer wall. At the upper left-hand corner of the figure a portion of a transversely cut rachis is marked by an arrow. 

 (Camera lucida drawing by Wieland.) 



seen in the lower portion of the disk is shown in figures 73 and 75. Near the 

 disk insertion the bundle system is formed by two concentric rows of bundles. 

 Higher up, the pattern of the bundles as seen in figure 74 . r is more complex and 

 foreshadows the bundle patterns of the individual fronds of the disk. Still higher, 

 where the rachides are free, there is a more and more simple distribution, at last 

 becoming peripheral and, as shown in figure 74/;, in part comparable to Bowenia 

 (figure 32, 1). Although the disk bundle system has plainly arisen from a series 

 of typical fern-frond bundles, the original form of the latter is no doubt much 

 altered by the assumption of the cyclic arrangement and compression in what may 

 well be called a "flower bud." 



