32 2 TIIALLOPHVTES. 



doubtful whether the change in the external form proceeds more from the gonidia 

 or from the hyphic. This relationship, which, although both morphologically and 

 physiologically important, has not hitherto had sufficient attention paid to it by lichen- 

 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 {gs), 

 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 cf 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^ Vsnea 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 d^nse 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 hyphae 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 



