FUNGI.— LICHENS. 



117 



this case the thallus is said to be homoionie7-ous ; or the j^onidia are crowded together 

 in one layer (Fig. 76), so that the hyphal tissue is divided into an outer and an 

 inner or into an upper and a lower layer as the case may be ; the tissue of the 

 thallus is therefore stratified, and such Lichens are said to be hcterotnerous (Figs. 

 76 and 79). 



The mode of growth, the branching and outward form of the thallus may cither be 

 determined by the gonidia, so that the hyphae play only a subordinate part in the 

 formation of that body, or the hyphae determine the form and the mode of growth, 

 while the share of the gonidia in the formation of the tissue is subordinate. The 

 former case occurs in only a few Lichens, the latter is the ordinary mode of growth in 

 typical Hchens, especially in those of the heteromerous kind. In many homoiomerous 

 gelatinous Lichens (Fig. jj) it seems to be doubtful, whether the change in the 

 outward form proceeds more from the gonidia or from the hyphae. This relation 

 between the gonidia and the hyphae, which is morphologically and physiologically 

 important, will be made sufficiently clear by examination of Figs. 78 and 79. Fig. 78 

 is an optical longitudinal section of a branch of Ephebe pubeseens ; the large 

 gonidia are shaded, the delicate hyphae are indicated by the letter h. The branch 

 elongates by growth at the apex and by corresponding transverse division of a 



l-IG. 77. I.eptO!;iittii sciitiiiion, vertical bection of llie gelatinous thallus. magn. 550 times; an epiilermal 

 layer clothes the inner tissue, which consists for the most part of formless and colourless jelly, in which the 

 coiled strings of gonidia lie; single larger cells of the strings Ithe limiting cells) are of a lighter colour : between 

 them run the slender hyphae. 



gonidium gs, which is the apical cell of the branch ; the cells derived from the 

 gonidium at the apex divide in a plane parallel to the longitudinal axis of the branch, 

 and further divisions then take place in different directions, and thus groups of gonidia 

 are produced at a considerable distance from the apex of the branch. The slender 

 hyphae reach in the figure up to the apical cell ; in other cases they come to an end 

 some way below the apical gonidium, and only a few single filaments grow inside the 

 gelatinous envelope, which is evidently produced from the gonidia, and follow the 

 longitudinal growth of the branch. It is only at some distance behind the apex of the 

 branch that the hyphae send out lateral branches which penetrate between the gonidia 

 and gonidial groups by growing through their softened mucilaginous cell-walls. Thus 

 the whole form of the branch, its growth in length and thickness, is determined by the 

 gonidia ; the hyphae with their small number and delicacy scarcely cause any change 

 of importance either in the external form or in the interior structure of the branch. 

 The same thing appears plainly in the formation of the lateral branches of the thallus 

 of Ephebe pubescetis ; one of the outer gonidia elongates in a direction at right angles to 

 the axis of the main branch of the thallus and becomes the apical cell of the lateral 

 branch, producing new cells by transverse divisions, as is shown in Fig. 78, a ; branches 

 of the hyphae at that spot turn in the same direction and behave in the same way 



