322 
THALLOPHFTES. 
doubtful whether the change in the external form proceeds more from the gonidia 
or from the hyphae. This relationship, which, although both morphologically and 
physiologically important, has not hitherto had sufficient attention paid to it by licheH- 
ologists, will be made sufficiently clear by an examination of Figs. 216 and 217. In 
Fig. 216 is shown the longitudinal section of a branch of Ephebe pubescens ] the large 
gonidia are left dark, and the very fine hyphae are indicated at h. The branch increases 
at the apex by longitudinal growth and by transverse division of a gonidium (^j), 
which is here the apical cell of the branch. The cells produced from the apical 
gonidium afterwards divide parallel to the longer axis of the branch ; still later divisions 
are formed in different directions, and thus groups of gonidia arise at some considerable 
distance from the apex of the branch. The delicate hyphae are represented in our figure 
as reaching to the apical gonidium; in other cases they come to a termination at a 
considerable distance beneath it. Even in this case it is only a few single hyphae 
which follow the longitudinal growth of the branch ; these grow within the gelatinous 
envelope which is evidently derived from the gonidia. At a considerable distance from 
the apex of the branch the hyphae first put forth lateral branches which penetrate be- 
tween the single or grouped gonidia, forcing their way through the deliquescent mass of 
their gelatinous cell-walls. Thus the whole form of the branch, its growth both in 
length and thickness, is determined by the gonidia ; the hyphae, from their small number 
and their fineness, produce scarcely any essential alteration either in the external form 
or the internal structure of the branch. This is clearly shown also in the origin of 
the lateral branches of the thallus. One of the exterior gonidia lengthens in a direc- 
tion at right angles to the axis of the parent-branch, and becomes the apical cell of 
the lateral branch, producing at the same time new cells by transverse divisions, as 
is shown in Fig. 216, a. Branches of the adjacent hyphae turn in the same direction, 
and behave, in relation to the new apical cell, in the manner described above with 
respect to those of the primary branch. 
In a manner similar to Ephebe pubescens, Usnea barbata, a fruticose Lichen, also forms 
a much-branched fruticose thallus. The branches of the thallus here also elongate by 
apical growth (cf. Fig. 217, A)', but this is not brought about, as in Ephebe, by the 
gonidia, nor by a single apical cell. Each of the hyphae at the end of the branch, which 
are nearly parallel and approximate at the apex, elongates by the apical growth of its 
terminal cell, and thus they produce in common the apical growth of the branch ; this is 
followed further backwards by an intercalary growth, the result of the intercalary 
elongation of the hyphae and of the formation of new hyphae in different directions. 
The hyphae lie so close together near the apex that they form a compact mass without 
interstices ; it is only at some distance from it that the hyphal tissue is differentiated 
into a very dense cortex of fibres interwoven on all sides, an axial bundle of densely- 
crowded threads running in the direction of length, and a looser layer (the medullary 
layer) furnished with air-containing interstices. The point below the apex where this 
differentiation of the hyphal tissue begins is also that of the point of commencement of 
the gonidial layer, which consists of small roundish green cells, collected in small groups 
in consequence of multiplication by division ; and these groups themselves form a layer 
between the medullary and cortical layers [cf. Fig. 217, B, the transverse section). 
Below the growing apex of the branch of the thallus there are only single gonidia, 
by the division of which the cells of the gonidial layer are produced. It is evident 
therefore that in Usnea barbata the growth in length and thickness and the internal 
differentiation of the tissue depend entirely on the hyphae, and that the gonidia behave 
like foreign bodies in the hyphal tissue ; the formation of new branches proceeds also 
from the hyphae and not from the gonidia. The branching may be dichotomous ; and 
in this case the apical cells of the hyphse converge towards two nearly adjacent points, 
and then continue to grow in corresponding directions, so that the two equal branches 
form an acute angle. Adventitious branches arise laterally below the apex of the thallus, 
the cortical fibres forming at a particular point a new apex and subsequently growing 
