﻿TRUNK STRUCTURE. 



77 



of the medullary rays increase in size toward the cortical parenchyma, as in other 

 cycads. The heavier-walled elements of the phloem alternate with the thinner- 

 walled with marked regularity in the transverse section. A sufficient number of 

 well-cut tangential sections is not at hand to give all the structural details. 



THE MEDULLA. 



The preservation of the medulla is quite complete in most of the Black Hills 

 cycads. In nearly all the medulla is very large, and so far as known is wholly 

 without vascular structures. In a few of the trunks there is, however, in the middle 

 region a large conical cavity varying 

 in diameter from a third to more than 

 half that of the entire medulla, and 

 apparently due to the separation of 

 the parenchyma which once filled 

 the space by an excision zone. No 

 sections have been cut determining 

 this point, but these conical cavities 

 are doubtless due to the formation 

 of bands of internal periderm, such 

 as occur abnormally in the stems of 

 recent cycads. Similar medullar 

 cavities have been observed in the 

 Italian Cycadeoidese and thus ex- 

 plained by Solms-Laubach (22). 



No very distinct systems of 

 gum canals have been observed. In- 

 stead of gum canals there appear to 

 be very irregularly distributed large 

 secretory sacs, three or four times 

 the diameter of the cells of the 

 ground parenchyma and more or 

 less elongate. These may occur 

 tandem to each other, and especially 

 near the leaf bases there are canal- 

 like lengthenings. The sac contents 

 are usually dense, with brownish 

 staining like that of the cell walls. 

 Occasionally, however, there is a quite dark coloration as if during silicification 

 there had been left over a slight residuum of carbon derived from resinous contents. 

 In yet other instances the secretory sacs are filled with a distinctly vacuolated mass, 

 which may also be a secondary form of the original contents. 



In the cells of the ground tissue three kinds of bodies are to be observed. 

 Particularly in Cycadclla (S. 50) large numbers of starch grains are present, quite 

 filling the cells over large areas. Also, in the several sections of trunk 393, granules 

 or vacuoles, rather larger than starch grains, and of more variable size, are often 



Fig. 42. — Cycadeoidea Wielandi. 



Tangential longitudinal section through xylem. showing the 

 structure of the medullary rays. Drawn from a photograph 

 displaying the fact that the walls of the tracheids, owing 

 to the manner in which the scalariform markings are natur- 

 ally iron stained, do not appear so dense in longitudinal as 

 in transverse sections. (Cf. figure 43.) X 100. 



