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24o 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 onesided
increase of monophyllous species such as Umbilicariapustulata. Wainio 1
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: 2
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.
Parmeliaphysodes rarely forms fruits, but when growing in an atmosphere
8
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
from moist to dry conditions may have a harmful effect on the developing
ascogonium.
The perithecia of Pyrenula nitida are smaller on smooth bark 4 such as
that of CoryluS) Carpinus, 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 adaptation
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.
2
Nienburg 1908.
3 Metzger 1903.
* Bitter 1899.