58 



species of Stereocaulon; these are very irregular in size and form, 

 though they never become large ; the symbiotic algae belong to the 

 genus Sirosiphon ; in these forms I was unable to determine whether 

 they were organically connected with the lichen-thalli, or whether 

 they are simply small undeveloped specimens of Ephebe pubescens 

 living in mutualistic relationship with the Stereocaulon-thattus. It 

 is evident that further careful work must be done in regard to the 

 morphology and physiology of cephalodia in order to determine their 

 true nature. 



(3) Endotrophic Cephalodia. 



These cephalodia, as the name indicates, occur wholly within the 

 tissues of the lichen-thallus ; in this country they are typically devel- 

 oped in Sticta Oregana and I have studied them more particularly 

 in that species ; they belong to the cephalodia vera of Forssell, who- 

 has also given a very accurate description of the development and 

 morphology of this form. In the endotrophic cephalodia as they 

 occur in S. Oregana the infecting alga is likewise some species of 

 Rivularia; they differ structurally from the cephalodia of Pelti- 

 gera aphthosa in that the infecting algae are enclosed by the hyphal 

 tissue of the lichen in which they occur. 



The mature cephalodia appear as spherical projections either 

 above or below the thallus, varying in size from a pin-head to a pea. 

 They seem to project downward about as frequently as they do up- 

 ward, and may occur in any part of the thallus except near the 

 margin. As far as I have been able to observe, they begin their de- 

 velopment in the medullary layer of the thallus ; I am unable to- 

 state in what manner the algae gain entrance to this layer, nor has 

 the mode of infection been accurately observed by any one. The 

 fact remains that when the algae have gained access to the interior 

 they begin to divide, increasing in number quite rapidly, and by a 

 mutualistic effect upon the surrounding hyphae these also undergo 

 more active development. As to the initiative which determines the . 

 formation of an inferior or superior cephalodium I am unable to 

 give any satisfactory explanation ; it may depend upon a difference 

 of resistance offered by the two surfaces : that is, if development be- 

 gins nearest the lower cortical layer it will project downward, if 

 nearer the upper cortical layer it will project upward. Both forms 

 are structurally much alike. In the superior form the upper corticaL 



