I40 OSMUNDACEAE [ch. 



(3) A coniitiuous xylevi-rlng surrounds a ce?ilral pith. 



Osmiitidites Du7ilopi from the Jurassic of New Zealand possesses a continuous xylem- 

 ring surrounding a pith badly preserved, but probably parenchymatous. "Most of the 

 leaf-traces, if not all, depart without in any way interrupting the continuity of the xylem- 

 ring" {I.e. No. I, p. 760). 



(4) An ittterrupted xyle7n-ring sin-rounds pith only. 



In O. Dowkeri., from the same beds as O. Dtmlopi, the pith is well preserved, and is 

 surrounded by a xylem-ring which is broken up by xylic gaps into about 30 distinct strands. 

 O. Gibbeana is similar, but the strands number about 20. O. Kolbei from the Neocomian 

 (Wealden) of Cape Colony supplies the transition from the inner xylem to a true paren- 

 chymatous pith by possessing a "mixed pith" with wide tracheides. It thus appears that 

 the establishment of a true pith and the interruption of the xylic-ring date from early 

 Mesozoic times. 



(5) An ititerrupted xyleni-ring with internal endodermis and phloem. 



This is first seen in Ostnitndites skidegatensis from the Cretaceous Period. The large 

 stele, 24 mm. in diameter, has some 50 strands surrounding a large pith partly sclerotic, 

 which becomes continuous with the cortex at each leaf-gap. No layer resembling an 

 endodermis has been distinguished, and it is practically impossible to set a definite limit 

 to the stele {I.e. No. i, p. 771). But the most remarkable feature of this fossil is the internal 

 phloem which lines the inner surface of the ring, and is connected with the outer through 

 each leaf-gap. The departure of the leaf-trace interrupts the continuity of the whole 

 vascular ring {I.e. p. 770). The interest of this is increased by the fact that a similar internal 

 phloem exists in O. ci?tnamoinea, but with a well-defined internal endodermis (Fig. 427, 

 p. 136). In this plant, however, as also in Todea hynienophylloidcs., the internal phloem 

 is local rather than general throughout the stem (Vol. I, p. 134). 



(6) A dictyostelie ring., with cortex and pith continuous through each leaf-gap: the 

 ineristeles surrounded by endodermis. 



This is seen in the largest of all the known Osmundaceae, Osnnendites Carnieri., from 

 Paraguay, of late but uncertain horizon {I.e. No. v. p. 475). The stele is 35 mm. in diameter, 

 with 35 distinct strands of xylem of the Osmundaceous type. It is delimited by a layer held 

 to be endodermis, while at the departure of each leaf-trace there is a wide gap, giving 

 continuity between cortex and pith. The phloem is badly preserved, but it seems probable 

 that it followed the endodermis round the inner surface of each meristele. This appears to 

 be an advance in a very large Osmundaceous type along lines of its own to dictyostely. 



The leading facts relating to the fossil Osmundaceae may be tabulated as on p. 141. 



From the facts contained in this table it follows: (i) that though there is no constant 

 numerical proportion between size and the complexity of vascular construction still, speaking 

 generally, the small fossils are simple, and the largest the most complex ; (ii) that the 

 progression in time runs substantially parallel with the progression in stelar complexity 

 up to the Mesozoic age, at which time its highest point had been attained for the Family ; 

 and (iii) that the ordinary type of modern Osmundaceous structure was acquired in 

 Jurassic time, and is there represented by examples which, though small as compared 

 with other related fossils, correspond very nearly to the dimensions of the living species. 

 But the largest fossils referred to the family became actually dictyostelie, a state not 

 represented in living Osmundaceae. The phyletic application of these facts and conclusions 

 is deferred till all the data material for that discussion have been considered. 



The origin of the leaf-trace and its ontogenetic expansion into the charac- 

 teristic C-shaped meristele has already been described for Thamnopteris 



