THE DICTYO-XYLIC RING 



133 



and by Faull, reflects the probable phylesis. The young plant has a solid 

 xylem-core, from which the first leaf-traces spring in the manner usual in 

 protostelic Ferns. But as the later traces depart parenchyma appears among 

 the stelar tracheides. Higher up a permanent column of pith is established 

 at the centre of the wood. From such ontogenetic facts it may be concluded 

 that the protostele was primitive for the living Osmundaceae, and the medul- 

 lated stele derivative. In this medullated region the xylem-ring is interrupted 

 above each leaf-trace by a xylic gap. Through it the pith is connected by 

 a parenchymatous ray with the parenchymatous sheath outside the xylem ; 

 and since the leaves are numerous, and closely placed, these xylic gaps 

 overlap, and produce the characteristic dictyo-xylic state of the adult stem 

 of Osmicnda (Fig. 120, p. 127). All these arrangements are still intra-stelar, 

 for outside the xylem-sheath lie 

 the continuous cylinder of the 

 phloem, the pericycle, and finally 

 the endodermis. In the adult the 

 latter remains unbroken at the 

 departure of each leaf-trace. 



All this appears intelligible 

 as an upgrade development from 

 the protostele, following on the 

 ontogenetic enlargement of the 

 shoot. The result is that in the 

 adult shoot there is a bulky 

 central column of storage-pith, 

 sometimes partially sclerotic, sur- 

 rounded by a dictyo-xylic ring, 

 with peripheral phloem, peri- 

 cycle, and endodermis. Outside 

 this again is a densely starchy 

 cortex. The conducting tissues thus lie between two storage-tracts, both of 

 which are ventilated by intercellular spaces, though the conducting tract 

 which completely separates them is not. It is delimited by an external 

 endodermis, but there is no internal endodermis in ordinary stems of the 

 living Osmundaceae. 



An internal endodermis does, however, appear occasionally in the living 

 Osmundaceae. It has been noted locally in some few specimens of Todea 

 hyinenophylloides as a discontinuous sheath, with its suberisation not confined 

 to one layer of cells. There is no internal phloem associated with it. (Seward 

 and Ford, Trans. Linn. Soc. vi, 1903.) But in adult stems of Osmunda 

 cinnamomea both endodermis and phloem are found in some cases within 

 the ring of xylem : in other specimens of the species there is an internal 



Fig. 126. Stele of a full-grown stem of Osiminda cin- 

 namotnea, from a photograph by Gwynne-Vaughan. 

 For details see text. ( x 25.) 



