Relation of Fungi to other Or i^ani^ms. A. Lorrain Smith. 27 



reaps most benefit from the alliance, since the carbohydrates 

 so necessary to the continued life of the hvphai are always 

 being prepared and yielded up by the green cells which 

 are eventually destroyed, with but little compensating 

 advantage' at any time. There is however a very delicate 

 interchange of food material going on between the symbionts, 

 it is not a simple tale of mere plimder. Pure cultures of 

 allied algae and even of lichen gonidia — the term applied 

 (o algae within the lichen — have been made by various 

 workers, and they have proved, that on certain substances 

 such as vegetable acids, the green cells can live and thrive 

 and form not only carbohydrates but even chlorophyll in 

 the dark, and in the absence of carbon dioxide. Such 

 substances as w^ell as nitrogenous material are supplied more 

 or less abundantly through the agency of the lichen fungus 

 and contribute to the support of the algal partner, though 

 the first and most fruitful soinxe of carlion supply must be 

 through the natural channel of assimilation from the 

 atmosphere. 



Lichen .lli^ae and Fungi, and their method oj associidion. 

 Lichen Fungi belong to the Ascomycetes, with the 

 exception of a few Basidiomvcetes which form lichens in 

 tropical countries. Lichen hyph^e are filamentous, branched 

 and septate with apical development and frecjuently Avith 

 thickened gelatinous walls, which absorb and retain 

 moisture. 



The algal cells or gonidia belong to the aerial 

 Myxophyceae or Chlorophyceae, and are one-celled organ- 

 isms or at most shortly filamentous. They increase within 

 the lichen plant by cell-division or by sporulation, though in 

 free conditions reproduction may be by zoospores. 



The Myxophyceae or blue-green algae are mostly 

 distinguished by their colouring and by the gelatinous cell- 

 sheaths : the hyphae as a rule permeate the sheath, leaving 

 the cells intact. This %ype of contact prevails in Gloeocapsa 

 and allied algal forms; in Rivularia, Scytoncma and 

 Stigonema algae which grow in tufted filaments, the 

 hyph^ travel along their sheaths sharing in the apical de- 

 velopment of the alga and the lichen formed is, like those 

 algae, of tufted form. Xostoc, a coiled filamentous form, 

 yields abundant mucilage which, in Collemaceae, is threaded 

 by the lichen hyphai. Certain cells of the Nostoc chain, 

 however, were shown by Bornet to be attacked and entirely 

 destroyed by the fungus, thus breaking the chain to short 

 lengths which continue a healthy existence. 



In the Chlorophyceae, which are mostly non-gelatinous, 



