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 of such a branch, as it rapidly elongates (Figs. 9 and 10), 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 1220 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. 10 and n). 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 



