908 MR A. C. SEWARD AND MR A. W. HILL ON THE 



to the stem having been preserved in the position in which it is represented in fig. 1. 

 The tree may have been killed by the effects of neighbouring volcanic eruptions ; and 

 after the decay and removal of the inner and more delicate cortical tissues, the hollow 

 shell was filled with volcanic ash. Silica was the petrifying medium, and the preserva- 

 tion of the tissues is unusually perfect : in the central cylinder there are numerous 

 spherulitic patches scattered through the matrix ; it would seem that silicification was 

 first completed round definite isolated centres, and this was followed by a secondary 

 crystallisation in the matrix, which partially obliterated some of the more delicate 

 structural features. 



Several of the photographs show the concentric bands of the circular areas of crystal- 

 lisation — as in PI. I. fig. 5 and PI. II. figs. 13-15.* 



The shell of the bark consists in the main of secondary tissue, from 7 to 8 cm. in 

 thickness, for which we shall use Von Mohl's term — Phelloderm ; externally this is 

 broken up into irregular patches, as seen in the lower part of fig. 1, by numerous 

 longitudinal fissures, some of which were probably formed during the life of the tree. 

 Decay, previous to the introduction of the silicifying solution, removed the greater part 

 of the cortical tissues, but it will be shown that the lost tissue no doubt consisted of 

 thin-walled parenchyma. The central cylinder has a diameter of 6 "5 cm. ; the centre is 

 occupied by concentric bands of silica, seen as a white spot in fig. 1 ; surrounding this 

 is a band of unequal breadth of pith- tissue (fig. 1) about 11 mm. in breadth, suc- 

 ceeded by a ring, 3 mm. in width, of primary wood x' ; beyond this is a broad zone of 

 secondary wood x" , 2*2 cm. in diameter (vide also fig. 33, PI. IV.). External to the 

 secondary wood some of the tissues have fortunately been clearly preserved. 



A. Description of the Tissues. 



i. Pith.t — The band of pith (fig. 1, PI. I.) has an irregular inner margin, where 

 it is succeeded in the centre by concentric layers of silica ; probably the pith was not quite 

 solid in the old part of the living tree. The cells of this region are parenchymatous in 

 form, and vary considerably in size ; those in contact with the innermost tracheids of 

 the primary wood have a more regular arrangement and a more uniform size than those 

 towards the interior (PL I. fig. 4, PI. IV. fig. 25). As we pass towards the centre of the 

 pith, the cells become more irregular in shape, and many of them have the form of long 

 filaments, which show a tendency to grow in a radial and more or less vertical direction, 

 increasing in length by apical growth, and dividing by transverse walls. The general 

 appearance of the parenchyma of the pith, especially the more internal portions, sug- 

 gests an actively-growing stout hyphal tissue, comparable to the hyphse in the 



* Vide Seward, lac. cit. 



t The term "pith" is used for the central tissue of the stele as a matter of convenience, but it would, perhaps, 1 i 

 hotter to adopt the expression " central conjunctive" [Flot (93)]. Bertrand has discussed the morphological nature "i" 

 this region in his paper on L. Hnrcourtii [Bertrand (91)]. 





