TEN YEARS' PROGRESS IN LICHENOLOGY. 281
The authors sum up the conditions under which the lichens exist
in the following paragraph.
"Like the scouts of an army these outposts of vegetable
life lead a precarious existence and they 'take cover' behind
any prominent object, especially on the leeward side. The
attitude of many of the species may be described as one of
'crouching' to obtain shelter from the wind."
O. V. Darbishire (14) wrote on the lichen thallus as adapted
to the nature of the substratum on which it is found. Many
crustaceous forms growing on the bare surface of a rock, as
Rhizocarpon confervoides and R. geographicum, have, rising from
the hypothallus, which firmly adheres to the rock, a number
of many-sided irregular columnar areas separated from one
another by numerous microscopic clefts. The outer surface of an
area is covered completely by a regular fungal cortex, but this
does not continue down the sides of the clefts, where the gonidia
are almost at the surface. During a period of dry weather the
clefts widen, but immediately on the fall of rain the water is
drawn in between them ; the thallus swells up and the openings
are closed, so that the upper surface becomes practically entire
and evaporation is for the most part suspended.
As an example of lichens that grow within minute cavities
of rocks Verrucaria calciseda is taken as a type. This lichen
eats its way into calcareous rocks, forming minute pits in which
perithecia are developed. There are several Verrucarias and
allied species of a similar habit.
Corticolous lichens are grouped in a corresponding way,
viz. those growing on the exterior of the bark, and those appear-
ing under or within the bark.
An interesting feature of this paper is the series of excellent
photographs by which it is illustrated.
Papers treating of the Morphology and Physiology of
lichens must be considered with those treating of the question
of Symbiosis. These have been few in number. Paulson and
Hastings (23) discuss the relation between the alga and fungus
in the lichen thallus in cases where the alga belongs to the Chloro-
phyceae, and conclude that penetration of the algal cell by the
fungal hypha occurs very seldom, if ever. Any theory of
parasitism, or helotism, based upon the fact of penetration,
has very little indeed to support it. The authors note (1) that