ZYGOPTERIS 315 



solid vertical angles of the vessels. This unexpected 

 result, which was first suggested by observations on 

 fossil Osmundaceae, goes to show that the true scalari- 

 form duct, far from being primitive, is a highly modified 

 form of tracheal organ. 



The protoxylem in Z. Lacattii occurs, as already 

 mentioned, on the outside of the two lateral bands of 

 wood, forming two groups on each side. The phloem 

 surrounds the wood completely, and contains large 

 elements, presumably the sieve-tubes ; the whole bundle 

 is enveloped by a dense sheath. The inner parenchy- 

 matous zone of the cortex is traversed by numerous 

 sacs, supposed to have been of a secretory nature. 

 The outer layers of the cortex form a continuous zone 

 of thick-walled mechanical tissue. 



The leaf was obviously a compound one, for lateral 

 bundles are given off from the side-pieces of the main 

 strand, starting from the points where the spiral 

 elements are placed. There were thus four rows of 

 appendicular bundles in all, two on each side of the 

 petiole. The significance of this peculiar arrangement, 

 which was widely distributed in this family, is not yet 

 understood. In some cases there is evidence that the 

 smaller lateral strands supplied the " Aphlebiae," but in 

 other cases they certainly passed into branches of the 

 rachis. Mr. Tansley, 1 on theoretical grounds, suggests 

 that " this tendency to radial organisation of the frond 

 may perhaps be regarded as a relic of the time when 

 the structure of the fronds of Ferns was but little 

 differentiated from that of the stem." 1 It must be 



1 " I. Lectures on the Evolution of the Filicinean Vascular System. II. 

 The Botryopterideae," New Phytologist, vol. vi. 1907, p. 65. 



