Recent Works on Lichens. A. Lorrain Smith. 201 
obscure the sections were wholly dark. The differences were so 
constant as to be of specific importance. 
Nienburg (1919) has attacked the debated subject of the rate 
of lichen increase in size. He watched lichen sporlings in nature 
for a number of years and found that light was more favourable 
to growth than shade, which suggests that the hyphae respond 
to increase of gonidia. Taking Parmelia furfuracea he figures 
after one year’s growth minute lobes which in three years had 
reached a length of 1-75 mm. in the shade and about 7:25 mm. 
in full light. Finally after eight years the maximum size attained 
was about 22mm. x 35mm. These observations agree fairly 
well with the rate of growth previously observed in foliose 
lichens, though some other types of lichens increase at a more 
rapid rate. During the same research he also established de- 
finitely that lichens are phototropic: that they grow towards 
the light and even manage to orient their surfaces so as to gain 
the fullest amount of sunlight though in this respect lichens are 
not all equally affected. 
Strato (1921) made a series of observations on growth and 
regeneration in the thallus of Peltigera canina, and he comments 
also on the importance of light. As light is a necessity for the 
life of the gonidia, so it is one of the essentials of thallus re- 
generation. Growth in the thallus of Peltigera takes place at 
the circumference; isidia as outgrowths from the thallus are in 
respect of increase terminal bodies: when the thallus is injured 
isidia are frequently formed, being induced by the urgent growth 
of the algae. The cortex of the thallus is covered with felted 
hair and is not able alone to initiate new growth. Moisture as 
well as light is an essential of growth and the portions of the 
thallus under experiment would not “regenerate”? under a 
moist bell glass unless on a substratum of earth or moist clay. 
Tobler (1921) draws attention to the few lichens now found 
growing in the Wolbeck Zoological Garden in Westphalia com- 
pared with the large number that were recorded (1856-85). He 
suggests as a reason for the decrease that lichens are very 
sensitive to change of environment, and chief among the changes 
that have taken place is the increase of moisture to a degree un- 
favourable to their growth. The trees in the enclosure have 
increased, some of them have been felled, and the lichens mean- 
while have decreased. 
ECOLOGY. 
A number of important papers have appeared on some aspect 
or other of lichen habitats and lichen ecology. One of the most 
interesting of these is on ‘‘ Nitrophilous Lichens” by Sernander 
(1912). The influence on certain lichens of an abundant supply 
of nitrogen has long been recognized, but Sernander is the first 
