IOO 
Oliver. — On the Obliteration of 
into the cortical tissue, the radial rows of sieve-tubes being 
continued almost imperceptibly into rows of parenchyma- 
cells — isodiametric in transverse section — but longitudinally 
elongated. This parenchyma is thick-walled and strongly 
pitted, and its walls show very well layers of stratification 
when mounted in glycerin. 
Towards the periphery of the stem this tissue passes over 
into a much thinner-walled parenchyma, with cells arranged 
in radial rows, and more or less radially elongated. This 
tissue is formed by the active division of a meristem or 
cambial layer arising only a few layers below the epidermis. 
In it are seen the young mucilage-ducts developing. They 
arise in a zone all round the stem at the junction of the inner 
thick-walled cortex and the outer cortex, which is formed 
from the cambial layer. 
The limiting layer or epidermis consists of small and very 
closely-packed somewhat columnar cells. The outer wall is 
thick, with a well-developed cuticle. These cells, alone in the 
stem, contain the chromatophores with the ordinary brown 
colouring matter of the Laminarieae. The mucilage-ducts 
do not as yet contain mucilage ; their origin is schizogenetic, 
and recalls forcibly that of the resin-passages from the 
cambium of Hedera helix. 
In older stems, diameter about 6\ mm., a broad zone 
of secondary cortex has been formed, and the mucilage-ducts 
constitute a ring at the outer limit of the inner cortex. They 
have increased in size much, and their cavities are filled up 
with a mucilage staining brown in iodine. 
In adult stems — about 8| mm. in diameter — these ducts 
are seen half-way between medulla and epidermis. They 
branch freely in the outer cortex, never in the inner, and old 
ones often show an interesting development of thyloses , due to 
the ingrowth, and subsequent division in the lumen, of the 
secreting cells. It should be pointed out that the ducts are 
not everywhere lined with secreting cells, only at special 
circumscribed areas ; often small chambers lined with secre- 
tory cells occur, these open into the ducts. The structure 
