318 



FILICALES 



[CH. 



and a pericycle which abuts on the parenchymatous ground-tissue without 

 any definite endodernial layer. A peculiarity in the vascular strands is 

 that the first-formed elements of the phloem lie close to the edge of the 

 xylem, the metaphloem being therefore centrifugal in its development. 

 The ground-tissue is devoid of mechanical tissue and is penetrated by 

 roots, a few of which arise from the outer vascular strands while others 

 force their way to the surface from the more internal dictyosteles. Leaf- 

 traces, consisting of several strands, are given off from the outermost 



B 



242. 



A — D. Aiii/iiipleris evecta. 



A. Apex of sporaQKium showing "annulus." 



B. Sori. 



C. Sporangium. 



D. Section of sporangium, showing the two lateral bands of thick- 

 walled cells. 



E. Daiiaea : a, roof of sjnangium, with pores; h, sporangial 

 cavities ; v, vascular bundle ; i, indusium. 



(D, after Zeiller.) 



cylinder and a segment of the second dictyostele moves out to fill the 

 gap formed in the outermost network, while the gap in the second cylinder 

 receives compensating strands from the third. A few layers below the 

 surface of the petiole there is a ring of thick-walled elements («, fig. 243), 

 and in both petiole and stem numerous mucilage ducts and taimin-sacs 

 occur in the ground-tissue. It lias been shown by Farmer and Hill' that 

 in some of the vascular strands in an Angioptvris stem a few secondary 



1 Farmer and Hill (02) PI. xviii. figs. 2G, 28. 



