FUNGI. 267 



apex of the branch the hyphae first put forth lateral branches which penetrate between 

 the single or grouped gonidia, forcing their way through the deliquescent mass of 

 their gelatinous cell-wails. In this manner the whole form of the branch, its growth 

 both in length and thickness, is determined by the gonidia; the hyphas, 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 of Ephebe pubescens. One of the exterior gonidia 

 lengthens in a direction vertical 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 divi- 

 sions, as is shown in Fig. 190, a. Branches of the hyphae which run into this cell 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. 191, A); but this is not brought about, as in Ephebe, by the 

 gonidia, nor by a single apical cell. The hyphae at the end of the branch which are 

 nearly parallel and approximate at the apex, elongate each by the apical growth of its 

 terminal cell, and thus produce in common the apical growth of the branch ; this is 





m 



FIC. 192.— Verti-al section of the gymnocarpous apotlicciuin n{ Aitaptychia ciliaris (X about 50) ; h the Iiymciiiimi, j' sub- 

 hymenial layer and excipuUiiii ; all the rest belongs to tlie thallus ; -,n its medullary layer, r its cortex, g its gonidia ; at ^i" the 

 thallus forms a cup-shaped rim round the apotheciuin. 



followed further backwards by an intercalary growth, the result of the intercalary 

 elongation of the hyphae and of the formation of new branches 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 v.-ith 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 their increase by division. But these groups themselves lie in a layer 

 between the medullary and cortical layers {cf. Fig. 191, 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 in the gonidial layer subsequently increase. It is evident 

 therefore that in Usriea 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 



