336 DR. E. DE FRAINE ON THE 
The pericycle, composed of two or three layers of cells, is surrounded by 
an endodermis, which forms the “limiting layer” between the stem tissue 
and the foliar sheath ; primary cortex is absent, but a more or less extensive 
secondary cortex may be produced by the activity of the secondary 
cambium. 
. 
Secondary Structure, 
The eambium of the primary bundles is funetional for a very short time 
only, and the zone of secondary tissue which forms the bulk of the mature 
stem, is due entirely to the activity of a secondary meristem. At a very 
early stage a eambium arises in the pericycle immediately opposite the 
primary bundles (PI. 15. fig. 10); these isolated ares of meristematic tissue 
are rapidly joined up, so that a complete ring of cambium surrounds the 
primary bundle-system. On its inner side this meristematic layer gives rise 
to strongly lignified, prosenchymatous ground-tissue, and at certain points 
groups of vessels. Where these latter are formed, the cambium on its outer 
side produces a group of phloem elements ; as the neighbouring cells continue 
the formation of the lignified ground-tissue on the inner side, this phloem 
bundle (on a radial line with the vessels) soon becomes embedded (Pl. 15. 
fig. 11), its eambium then ceases its activity, a new cambial are joins up: 
the broken ring outside the embedded phloem, and the formation of lignified 
ground-tissue is resumed. The collateral bundles thus formed become com- 
pletely surrounded by the fibres of the intermediate tissue (РІ. 16. fiy. 12). 
As this anomalous secondary growth proceeds with comparative rapidity, an 
old stem shows a considerable thickness of lignified tissue in which the 
collateral vascular bundles are arranged in more or less regular concentric 
zones. 
The outer side of this secondary cambium is usually inactive, or at most 
gives rise to two or three layers of cortical cells. An exception to this 
occurs in the regions associated with the formation of aerenchyma, but will 
be described later in connection with that tissue. 
With regard to the anomalous secondary thickening in the stem and root 
of Salicornia, we find ourselves in complete agreement with the account 
given for the root by Fron * and [ог е stem by Solereder f. The account 
of the phenomena given by De Bary } for “S. herbacea” differs in some 
respects, for he describes an extrafascicular cambium which “remains per- 
manently active, and forms on its inner side alternately collateral vascular 
bundies and intermediate tissue, on its outer side a thin layer of bast, 
consisting only of parenchyma, or even no bast at all.’ In the rhizomes of 
* Ann. Sc. Nat. sér. 8, Dot. ix, (1899) p. 157. 
T Syst. Anat. Dicot., Engl. Trans. 1908, ii. p. 1127 ete. 
f Comp. Anat. Phaner. and Ferns, Engl. Trans, p. 590 
