CHAPTER II.—DIFFERENTIATION OF THE THALLUS.—-MYCELIAL STRANDS. 25 
The cells of the hyphae are 2-4 times longer than broad, and have a firm brown 
membrane and a polygonal transverse section in conformity with the absence of 
intercellular spaces; the cells of the outer layers are narrower than those of the inner 
and have much thicker walls. The stratification of their membranes becomes more 
conspicuous when they are treated with potash. 
The medulla consists chiefly of slender tough longitudinal hyphae about 1.5 mm. 
in thickness, which form acute angles with one another as they interweave, and have 
air in their interstices. Their membranes are comparatively firm; septation and 
branching are rarely seen in full-grown specimens. The slender medullary hyphae 
are in connection with the innermost layers of the rind; longitudinal sections show 
them arising as numerous branches from the cells of these layers, and making their 
way between them or running directly from them in oblique or transverse course to 
the medullary tissue. The longitudinal arrangement of the layers bordering on the 
medulla is thus rendered irregular to a degree which varies in each specimen and is 
sometimes considerable. . 
I have observed the development of the subterranean strands on adventitious 
branches, which it is not difficult to obtain from old specimens if cultivated in a damp 
chamber. The apex ofsuch a branch, as it rapidly elongates (Figs. 9 and ro), is conical 
in form and colourless for a distance of some millimetres. It consists of a weft of 
delicate hyphae rich in protoplasm, the terminal branches of which form at the apex a 
loosely tangled tuft rendered slimy by the gelatinous swelling of the membranes. 
The apical tissue is continued downwards in the periphery of the branch into the 
gelatinous felt which covers it and which will be described presently, and in the 
middle into a short-celled irregular tissue of interwoven hyphae without interspaces, 
which forms the real conical growing point of the body of the strand. Active 
meristematic cell-multiplication, which cannot be followed in detail on account of the _ 
close interweaving of the hyphae, takes place in the uppermost region ; close beneath 
this, where the strand begins to grow broader, there is partly elongation and extension 
of the elements of the tissue, partly formation of new elements. The former affects 
first and chiefly the axile portion of the strand, which occupies a third part of the 
total thickness; its cells subsequently undergo a few divisions close beneath the 
growing point and expand rapidly to a breadth of about 12-20 p» and 2-8 times that 
length ; they continue thin-walled, are filled chiefly with hyaline cell-sap and are 
arranged in straight longitudinal rows. They diminish gradually in breadth as they 
approach the peripheral tissue (compare Figs. ro and 11). An evident elongation 
of the cells takes place in the peripheral tissue, which serves to show more clearly their 
arrangement in longitudinal rows, but the increase in breadth is only small. As the 
circumference of the strand increases with every successive transverse section from 
the apex of the cone to the fully formed cylinder, and the hyphae are in close contact 
with one another without interstices, there must necessarily be an interpolation of new 
hyphal branches between those already formed. 
The development of the definitive structure of the strand begins with the passage 
into the ultimate cylindrical form. The increase in the breadth of the large axile cells 
ceases near the apex, where the peripheral layers of the circumference ‘increase con- 
siderably ; the consequence is that the axile cells are torn from one another especially 
laterally, and intercellular spaces are formed between them, which serve from the first 
