72 Seward. — A Contribution to our 
tissues have been torn, apparently along the line of cambium, 
which gave rise to the centripetal xylem. Internal to the 
phloem a patch of medullary parenchyma is preserved, con- 
taining a sclerous nest and several black patches of secretory 
cell-contents. 
Reserving for further consideration the nature and relations 
of the centripetal and centrifugal xylem, we may pass on to 
describe the pith. In Plate V, Fig. 2, the lighter portions in 
the pith mark the spots where the tissues have been destroyed 
and their place taken by mineral substance ; the dark patches 
represent parenchymatous tissue containing numerous sclerous 
nests, which appear as dark dots in the photograph, and 
secretory cells. On examining transverse and longitudinal 
sections of the pith under a low magnifying power, one sees 
distinct signs of active merismatic division as expressed by 
the arrangement of parenchymatous cells in regular serial 
rows. Some of the darker portions in Fig. 2 consist of 
secondary parenchyma developed from a zone of meristem 
situated on the external limits of the darker-coloured and 
rounded outlines of the tissue-patches. In Fig. 14 a portion 
of the pith is represented, very slightly enlarged ; immediately 
internal to the centripetal wood there are patches of paren- 
chymatous tissue containing secretory cells and three sclerous 
nests ; internal to this, and separated from it by an intervening 
band of mineral matrix, we have a mass of parenchyma of 
which a considerable portion is made up of a series of regular 
radiating rows of elements formed by cambial activity, the 
cambium being situated at the inner edge of the secondary 
tissue (c). This secondary medullary parenchyma presents 
the same appearance in longitudinal as in transverse sections. 
In describing the occurrence of anomalous xylem in the 
pith of Lyginodendron Oldhamium , Williamson and Scott 1 
point out that in some cases the cambium gives rise to secondary 
parenchyma instead of xylem and phloem. The secondary 
parenchyma in the pith of Lyginodendron robustum is, on the 
1 Loc. cit., p. 722. See also the reference on p. 723 to ‘Some specially com- 
plicated forms of anomalous tissue in the pith of Lyginodendron Oldhamium.' 
