34 CONSTITUENTS OF THE LICHEN THALLUS 



Lindau' has described the association between fungus and alga in 

 Pertusaria and other crustaceous forms as one of contact only (Fig. 12). 

 He found that the cell-membrane of the two adhering organisms was un- 

 broken. Occasionally the algal cell showed a slight indentation, but was 

 otherwise unchanged. The hyphal branch was somewhat swollen at the tip 

 where it touched the alga, and the wall was slightly thinner. The attach- 

 ment between the two cells was so close, however, that pressure on the cover- 

 glass failed to separate them. 



Generally the hypha simply surrounds the gonidium with clasping 

 branches. Many algae also lie free in the gonidial zone, and Peirce= claims 

 that these are larger, more deeply coloured and in every way healthier 

 looking than those in the grasp of the fungus. He ignores, however, the case 

 of the soredial algae which though very closely invested by the fungus are 

 yet entirely healthy, since on their future increase depends in many cases 

 the reproduction of new individual lichens. 



In a recent study of a crustaceous sandstone lichen, " Caloplaca pyrdceal' 

 Claassen" has sought to prove a case of pure parasitism. The rock was at first 

 covered with the green cells of Cystococcus sp. Later there appeared greyish- 

 white patches on the green, representing the invasion of the lichen fungus. 

 These patches increased centrifugally, leaving in time a bare patch in the 

 centre of growth which was again colonized by the green alga. The lichen 

 fruited abundantly, but wherever it encroached the green cells were more 

 or less destroyed. The true explanation seems to be that the green cells 

 were absorbed into the lichen thallus, though enough of them persisted to 

 start 'new colonies on any bare piece of the stone. In the same way large 

 patches of Trentepohlia aurea have been observed to be gradually invaded 

 by the dark coloured hyphae of Coenogonium ebeneutn. In time the whole of 

 the alga is absorbed and nothing is to be seen but the dark felted lichen. 

 The free alga as such disappears, but it is hardly correct to describe the 

 process as one of destruction. 



This algal genus Trentepohlia {Chroolepus) forms the gonidia of the 

 Graphideae, Roccelleae, etc. It is a filamentous aerial alga which increases 

 by apical growth. In the Graphideae, many of which grow on trees beneath 

 the outer bark (hypophloeodal), the association between the two symbionts 

 may be of the simplest character, but was considered by Frank" to be of an 

 advanced type. According to his observations and to those of Lindau^ the 

 fungal hyphae penetrate first between the cells of the periderm. The alga, 

 frequently Trentepohlia umbrina, tends to grow down into any cracks of the 

 surface. It goes more deeply in when preceded by the hyphae. In some 

 species both organisms maintain their independent growth, though each 

 shows increased vigour when it comes into contact with the other. In some 



' Lindau 1895I. ^ pgij^e 1899. s Claassen 1914. * Frank 1876. » Lindau 1895. 



