-CHAP. II.—DIFFERENTIATION OF THALLUS.—COMPOUND SPOROPHORES. 55 
“ 
the apex of the pileus soon cease to lengthen; they become the tissue of the middle 
of the pileus, while as the margin advances the hyphae which run into it send out 
numerous straight or curved branches upwards and outwards, which in their turn 
soon cease to elongate and form the general tissue of the pileus (Fig. 24 4). Closely 
crowded branches from the under surface of the layer which runs into the margin 
grow at the same time and in the same centrifugal succession from a curved base 
perpendicular to the under surface of the pileus; these are the beginnings of the 
tissue that bears the hymenium and of the hymenium itself. They are at first of 
uniform length, and the surface of the hymenium is smooth at first, as Hoffmann 
rightly affirms in opposition to a former incorrect statement of mine, though it only 
continues so for a short time. The elongation of the hymenial hyphae which 
grow vertically downwards takes place in alternating radial bands in varying degree. 
In some it continues longer, and they project beyond the smooth surface as the trama 
of the lamellae, on which the hymenial elements arise in the position already described, 
advancing from the base toward the free edge. The hyphal extremities cease to 
elongate at an earlier period in the intervals between the lamellae, and become 
directly elements of the hymenium. 
During this growth by terminal and marginal formation of new constituents, the 
parts at a distance from 
the growing point or mar- 
gin enlarge by expansion 
of their cells, and the tis- 
sue which is at first uni- 
form is differentiated at 
the same time into the 
several layers of the ma- 
ture sporophore. It is 
readily observed that this 
process of expansion also 
advances in the stipe from 
below upwards, and in 
the pileus from the centre 
to the margin. To this ex- 

pansion of the originally FiG. 24. Agaricus (Collybia) dryophilus, Bull. Radial longitudinal section showing 
very small elements to the course of the hyphae. @ a quite young and entire specimen 1°3 mm. in height; first 
beginnings of the pileus. 4 older specimen with the pileus 2°5 mm. in breadth; ¢ piece 
several times their former of 2 lamella Slightly magnified. 
size is due in great part, 
especially in rapidly growing fleshy sporophores, that enlargement which may be seen 
with the naked eye. In Agaricus (Mycena) vulgaris for instance I succeeded in deter- 
mining, by measurement of the cells and counting their number on the transverse 
section, that the increase in length and breadth of the stipe, which becomes on an 
average 50-60 mm. long, from the time when its length was about 3 mm. and its 
cells could be exactly measured, must be almost exclusively due to extension of 
the cells. I obtained the same result in the case of Nyctalis parasitica ; the conclusion 
was similar in the case of Agaricus (Collybia) dryophilus, Bull., but less precise 
on account of the very unequal length of cells placed at the same height. Exact 
measurements can scarcely be made in the pileus owing to the curvatures and 
want of uniformity in the cells, but here too it is evident that there is an expansion 
of the tissue-elements, which often exerts considerable force and advances towards 
the margin. It appears to me to be doubtful whether there is also any formation 
.of new cells by transverse division of the primary cells of the hyphae and by pro- 
duction of new branches in parts removed from the margin. It does not take place in 
either of the two cases just mentioned, but they are too isolated to allow of our 
drawing a general conclusion from them. Branches often occur on hyphae which 
