266 



THALLOP BYTES. 



The mode of growth, branching, and external structure of the Lichen-thallus may 

 either be determined by the gonidia, the hyphae being concerned only in a secondary 

 degree in the construction of the substance, or it may happen that the hyphae determine 

 the form and mode of growth, while the gonidia have only a secondary share in the forma- 

 tion of tissue. The first mode occurs in only a few Lichens ; the other mode of growth 

 is the more common, and is that of the typical Lichens, especially of those that are 

 heteromerous. In some homoomerous gelatinous Lichens (as Fig. 189) it appears 

 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 lichen- 

 ologists, will be made sufficiently clear by an examination of Figs. 190 and 192. In 



Fig. 190.— a liranch of the tliallus of 

 Ephebe pitbesccjis ( x 550). 



Pig. 191. — Usnea harbata; A longitudinal section of a slender 

 branch, soaked in potash-solution ; B transverse section of an older 

 thallus-stem with the basal portion of an adventitious (or soredial) 

 branch sa (X300) ; s apex of the branch, r the cortex, x the axial 

 medullary bundle, m the loose medullary tissue, g the gonidial 

 layer. 



Fig. 190 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 fine hyphae are represented in our figure 

 as reaching to the apical cell ; in other cases they come to a termination at a considerable 

 distance beneath the apical gonidium. There are also 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 



