41 
THE SPORULATION OF GONIDIA IN THE 
THALLUS OF EVERNIA PRUNASTRI ACH. 
With Plate I. 
By Robert Paulson, F.L.S. 
Evernia prunasini, one of the most widely distributed of the 
British fruticose lichens, occurs frequently, sometimes abun- 
dantly, on trunks and branches of trees in woodlands and 
hedgerows; less often on rocks, walls, palings and bare sandy 
soil. Its much branched flaccid thallus is decumbent or pendulous 
in habit except in the early stages of growth when it is more 
or less erect. A form vetusa, recorded from a small number of 
localities, consists solely of dense tufts of erect fronds(r1), In 
colour it is grey-green above and paler or whitish beneath. The 
branches are compressed and are frequently white-sorediate at 
the margin, the broader, older ones, being reticulate on the 
upper surface and finely, longitudinally channelled below. They 
terminate in two flattened points, which, after a period of 
further growth, fork again in a similar manner and thus exhibit 
the characteristic appearance of dichotomy. The apothecia, 
rarely produced, are large, sub-pedicellate and dark brown. They 
sometimes have a disc measuring as much as I2 mm. across; 
the margin is prominent and involute. 
Before cutting the sections from which the preparations, to 
be described later, were made, small portions of the thallus were 
placed in aweak chromo-acetic fixing solution for at least twenty- . 
four hours; a longer period does no harm. After thoroughly 
washing for two hours, the material was passed into paraffin 
wax in the ordinary way. Heidenhain’s iron-alum-haematoxylin 
and Delafield’s haematoxylin were the staining reagents em- 
ployed. Permanent mounts were as a rule put up in Canada 
balsam, but these do not show the cell wall sufficiently defined 
for all purposes, so that when preparing gonidia in the various 
phases of division, it was found advisable to mount some 
sections in glycerine, by which method the newly formed cell 
wall of a daughter gonidium is rendered distinctly visible (fig. 2). 
The portion of the thallus most favourable for obtaining large 
numbers of actively dividing gonidia is that part situated at 
from four to five millimetres below the apex of a young branch. 
In early spring, and in showery weather after a prolonged 
drought, numerous dividing gonidia can be found in all growing 
parts. 
