244 PHYSIOLOGY 



of the trunk that faces the prevailing winds. Wiesner^ remarks that spores 

 and soredia would more naturally be conveyed to that side; but there are 

 other factors that would come into play: the tree and the branches frequently 

 lean away from the wind, giving more light and also an inclined surface that 

 would retain water for a longer period on the windward side''. Spores and 

 soredia would also develop more readily in those favourable conditions, 



In forests there are other and different conditions: on the outskirts, 

 whether northern or southern, the plants requiring more light are to be found 

 on the side of the trunk towards the outside; in the depths of the forest, 

 light may be reduced from -^ to ^^, and any lichens present tend to be- 

 come mere leprose crusts. Krempelhuber' has recorded among his Bavarian 

 lichens those species that he found constantly growing in the shade: they 

 are in general species of Collemaceae and Caliciaceae, several species of 

 Peltigera {P- venosa, P- horizontalis and P. polydactyld) ; Solorina saccata ; 

 Gyalecta Flotovii, G. cupularis; P armaria microphylla, P. triptophylla, P. 

 brunnea; Icmadophila aeruginosa, etc. 



B. Effect on Reproductive Organs 



In the higher plants, it is recognized that a certain light-intensity is 

 necessary for the production of flowers and fruit. In the lower plants, such 

 as lichens, light is also necessary for reproduction; it is a common observation 

 that well-lighted individuals are the most abundantly fruited. In the higher 

 fungi also, the fruiting body is more or less formed in the light. 



a. Position and Orientation of Fruits with regard to Light. 

 There is an optimum of light for the fruits as well as for the thallus in each 

 species of lichen : in most cases it is the fullest light that can be secured. 



Zukal* finds an exception to that rule in species of Peltigera: when 

 exposed to strong sunlight, the lobes, fertile at the tips, curve over so that 

 to some extent the back of the apothecium is turned to the light; with 

 diffuse light, the horizontal position is retained and the apothecia face up- 

 wards. In the closely allied genera Nephroma, Nephromium and Nephro- 

 mopsis, the apothecia are produced on the back of the lobe at the extreme 

 tip, but as they approach maturity the fertile lobes turn right back and they 

 become exposed to direct illumination. In a well-developed specimen the 

 full-grown fruits may thus become so prominent all over the thallus, that 

 it is difficult to realize they are on reversed lobes. In one species of Cetraria 

 (C. cuctcllata) the rarely formed apothecia are adnate to the back of the lobe; 

 but in that case the margins of the strap-shaped fronds are incurved and 

 connivent, and the back is more exposed than the front. 



In Ramalina the frond frequently turns at a sharp angle at the point of 



^ Wiesner 1895. ^ R. Paulson, ined. 3 Krempelhuber 1861. * Zukal 1896, p. m- 



