LICHEN GONIDIA 41 



c. EFFECT OF SYMBIOSIS ON THE ALGA. Treboux's observations how- 

 ever convinced him that the alga leads but a meagre existence within the 

 thallus. Cell-division the expression of active vitality was, he held, of rare 

 occurrence in the slowly growing lichen-plant, and zoospore formation in 

 entire abeyance. He contrasts this sluggish increase 1 with the rapid multi- 

 plication of the free-living algal cells which cover whole tree-trunks with 

 their descendants in a comparatively short time. These latter cells, he 

 finds, are indeed rather smaller, being generally the products of recent 

 division, but mixed with them are numbers of larger resting cells, com- 

 parable in size with the lichen gonidia. He states further, that the gonidia 

 are less brightly green and, as he judges, less healthy, though in soredial 

 formation or in the open they at once regain both colour and power of 

 division. Treboux had entirely failed to observe the sporulation which is so 

 abundant at certain seasons. 



Their quick recovery seems also a strong argument in favour of the 

 absolutely normal condition of metabolism within the gonidial cell; and 

 the paler appearance of the chlorophyll is doubtless associated with the 

 acquisition of carbohydrates from other sources than by photosynthesis. 

 There is a wide difference between any degree of unfavourable life-conditions 

 and parasitism however slight, even though the balance of gain is on the 

 side of the fungus. It is not too fanciful to conclude that the demand for 

 nitrogen on the part of the alga has influenced its peculiar association with 

 the fungus. In the thallus of hypophloeodal lichens it has been proved 

 indeed that the alga Trentepohlia with apical growth is an active agent in 

 the symbiotic union. Cystococcus and other green algal cells are stationary, 

 but they are doubtless equally ready for as many of them are equally 

 benefited by the association. Keeble 2 has pointed out in the case of 

 Convoluta roscoffensis that nitrogen-hunger induces the green algae to 

 combine forces with an animal organism, though the benefit to them is only 

 temporary and though they are finally sacrificed. The lichen gonidia, on 

 the contrary, persist for a long time, probably far beyond their normal 

 period of existence as free algae. 



Examples of algal association with other plants might be cited here: of 

 Nostoc in the roots of Cycas and in the cells of Anthoceros, and of Anabana 

 in the leaf-cells of Azolla, but in these instances it is generally held that 

 the alga secures only shelter. It was by comparing the lichen-association 

 with the harmless invasion of Gunnera cells by Nostoc that Reinke 8 arrived 

 at his conception of "consortism." 



d. SUPPLY OF CARBON. Carbon, the essential constituent of all organic 

 life, is partly drawn from the carbon-dioxide of the air, and assimilated by 



1 See Paulson and Hastings 1920. 2 Keeble 1910. 3 Reinke 1872. 



