224 PHYSIOLOGY 



Hesse, the erythrit ester of lecanoric acid (Cis H^ O7), a very frequent consti- 

 tuent of lichen thalli. It is certain that the interaction of both symbionts is 

 necessary for acid production. This was strikingly d^emonstrated by Tobler' 

 in his cultural study of the lichen thallus. He succeeded in growing, to a 

 limited extent, the hyphal part of the thallus of Xanthoria parietina on 

 artificial media; but the filaments remained persistently colourless until he 

 added green algal cells to the culture. Almost immediately thereafter the 

 characteristic yellow colour appeared, proving the presence of parietin, 

 formerly known as chrysophanic acid. Tobler's observation may easily be 

 verified in plants from natural habitats. A depauperate form of Placodium 

 citrinum consisting mainly of a hypothallus of felted hyphae, with minute 

 scattered granules containing algae, was tested with potash, and only the 

 hyphae immediately covering the algal granules took the stain; the hypo- 

 thallus gave no reaction. 



It has been suggested^ that when a decrease of albumenoids takes place, 

 the quantity of lichen-acid increases, so that the excreted substance should 

 be regarded as a sort of waste product of the living plant, "rather than as a 

 product of deassimilation." The subject is not yet wholly understood. 



d. Causes of Variation in Quantity and Quality of Lichen- 

 Acids. Though it has been proved that lichen-acids are formed freely all 

 the year round on any soil or in any region, it happens occasionally that 

 they are almost or entirely lacking in growing plants. Schwarz' found this 

 to be the case in certain plants of Lecanora tartarea, and he suggests that 

 the gyrophoric acid contained in the outer cortex of that lichen had, been 

 broken up by the ammonia of the atmosphere into carbonic acid and orcin 

 which is soluble in water, and would thus be washed away by rain. It has 

 also been shown by Schwendener* and others that the outer layers of the 

 older thallus in many lichens slowly perish, first breaking up and then peeling 

 off; the denuded areas would therefore have lost, for some time at least, 

 their particular acids. Fiinfstiick* considers that the difference in the presence 

 and amount of acid in the same species of lichen may be due very often 

 to variation in the chemical character of the substratum, and this view tallies 

 with the results noted by Heber Howe" in his study of American Ramali- 

 nae. He observed that, though all showed a pale-yellow reaction with potash, 

 those growing on mineral substrata gave a more pronouncedly yellow colour. 



M. C. Knowles' found that in Ramalina scopuloriim the colour reaction 

 to potash varied extremely, being more rapid and more intense, the more 

 the plants were subject to the influence of the sea-spray. 



Lichen-acids are peculiarly abundant in soredia, and as, in some species, 



^ Tobler 1909. 2 Keegan 1907. ^ Schwarz 1880, p. 264. * Schwendener 1863, p. 180. 



^ Fiinfstuck 1902. " Heber Howe 19 13. '' Knowles 1913. 



