240 PHYSIOLOGY 



was exposed in the open among other branches in a wood while snow still 

 lay on the ground. In a short time the fungus revived and before the end 

 of spring not only had produced a new hymenium, but enlarged its hymenial 

 surface to about one-fourth of its original size and had also formed one 

 entirely new, though small, sporophore. 



b. On General Development. Lichens are very strongly influenced 

 by abundance or by lack of moisture. The contour of the large majority of 

 species is concentric, but they become excentric owing to a more vigorous 

 development towards the side of damper exposure, hence the frequent one- 

 sided increase of monophyllous species such as Umbilicaria pusttilata. Wainio' 

 observed that species of Cladonia growing in dry places, and exposed to full 

 sunlight, showed a tendency not to develop scyphi, the dry conditions 

 hindering the full formation of the secondary thallus. As an instance may 

 be cited Cl.foliacea, in which the primary thallus is much the most abundantly 

 developed, its favourite habitat being the exposed sandy soil of sea-dunes. 



Too great moisture is however harmful: Nienburg' has recorded his 

 observations on Sphyridium (Baeomyces rufus): on clay soil the thallus was 

 pulverulent, while on stones or other dryer substratum it was granular — 

 warted or even somewhat squamulose. 



Parmelia physodes rarely forms fruits, but when growing in an atmosphere 

 constantly charged with moisture', apothecia are more readily developed, 

 and the same observation has been made in connection with other usually 

 barren lichens. It has been suggested that, in these lichens, the abrupt change 

 frorp moiat to dry.coifditions may have a harmful effect on the developing 

 ascogonium. 



The perithecia of Pyrenula nitida are snladiir.@n smo6th'T3ark*<steEh as 

 that of Corylus, Carptnus, etc., probably because the even surface does not 

 retain water. 



IV. ILLUMINATION OF LICHENS 



A. Effect of Light on the Thallus 



As fungi possess no chlorophyll, their vegetative body has little or no 

 use for light and often develops in partial or total darkness. In lichens the 

 alga requires more or less direct illumination; the lichen fungus, therefore, 

 in response to that requirement has come out into the open : it is an adapta- 

 tion to the symbiotic life, though some lichens, such as those immersed 

 in the substratum, grow with very little light. Like other plants they are' 

 sensitive to changes of illumination: some species are shade plants, while 

 others are as truly sun plants, and others again are able to adapt themselves 

 to varying degrees of light. 



1 Wainio 1897, p. 16. '^ Nienburg 1908. ' Metzger 1903. ' * Bitter 1899. 



