I\chiti())i oj Fu)i<^i to other ()r<^(()iisnis. .1. J.orrain Siiiilh . 29 



cells lliat lie called il a ' necral zone'; I'"lenkin llierefore 

 rejects the symbiotic tlieor\' and look's on the association 

 as one of modified parasitism. 



While not rejecting P^lenkin's facis, we would ])oint out 

 the necessary decav of all living' organisms, when \'ilalit\' 

 becomes exhausted. The fmif^us in the lichen thallus suffers 

 loss in the superficial layers owintr to the wear and tear of 

 life, and these losses are compensated for by new upward 

 growth from the gonidial region. In the same way the 

 lower algae have been gradually pushed down to positions 

 further away from the light : they may have reached and 

 passed their maximum of vitality and there is no doubt that 

 decaying algae would promptly be used up by the h\ph£c 

 as food. That cells in Ihe close grasp of the hyphcC remain 

 active and healthy is abundantly proved by soredial 

 development. Soredia consist of one or more algal cells 

 generally completely invested by actively growing thin- 

 walled hyphic. Itach one is a lichen thallus in miniature : 

 it represents a budding off fnjm the parent plant of the two 

 constituents. At certain areas of some thalli, breaks occur 

 in the cortex and soredia like a white powder escape into the 

 open. They are scattered and vegetatively reproduce the 

 lichen ; in many species thev are the chief form of 

 reproduction, both, the symbionts being ready for active 

 increase. 



We conclude therefore that in lichens there is a true 

 symbiosis or mutualism, by which the fungus has accjuired 

 so much vigour, that a whole new class of plants with 

 well-defined peculiarities of structure and of cell products 

 have been evolved. The alga has been less affected than 

 the fungus, but it has gained in capacity of endurance 

 against time and against unfavourable conditions, impossible 

 without the shelter of the thallus and in some cases has 

 attained to much greater size as in the Stigonema of Ephebe 

 or the TreiitepohUa of some species of Coenogonium. 



The conception of symbiosis is familiar to us in many 

 different cases, and fungi have adapted themselves to that 

 mode of life with very considerable success, all the more 

 easily that they do not make large demands for sustenance : 

 thev are able to live on an extraordinarily meagre diet, and 

 with the po.ssible exception of resting spores and stromatoid 

 formations, the}^ make no provision for storage. They 

 are the happv-go-luckv members of the plant kingdom which 

 vet have their appropriate place and function, though at limes 

 driven bv some need to become predator}- and therefore 

 enemies in the plant society. 



