262 DR LAUDER LINDSAY ON THE SPERMOGONES AND PYCNIDES 
GENUS V. PLACODIUM, DC. 
This genus is intermediate in character between Parmelia or Physcia, and 
Lecanora ; but its spermogones are those of the two former genera. The sper- 
mogones are usually papillz or tubercles, of various size and variously coloured, 
scattered about the periphery of the sub-foliaceous thallus, and on the convexities 
of the laciniz, just as in Physcia. They are isolated or grouped, more or less 
irregular in form frequently, especially when confiuent, often flattened on the 
apex. Sometimes they are scattered on separate sterile areolee, as is the case 
with the spermogones of many Lecanore and Lecidew. They are then punctiform 
and immersed, with a raised, light, thalline border, as in P. chalybeeum and P. al- 
phoplacum. The spermogones are, in a form of P. circinatum, pseudo-lecidine in 
appearance, and as large as apothecia. They are the largest spermogones, with- 
out exception, with which I am acquainted. They are large whitish or buff- 
coloured disks, destitute of an exciple, having frequently a ragged or notched 
edge. ‘They are generally semi-immersed in the limestone on which the plant 
grows. The internal structure and contents are those of the ordinary form of 
spermogone in this species. The colour of the spermogone is also whitish in P. 
chalybceum, 11 which the ostiole is of a pale-gray. In species with a reddish or 
yellowish thallus, just as in Physcia, the colour of the spermogone is orange-red 
or orange-yellow. This is the case in P. muscorum, P. callopismum, P. elegans, 
and other species. The colour is sometimes brownish or blackish; in P. candicans 
it is bluish. The ostiole is usually very minute and indistinct ; sometimes it is 
fissured irregularly. The greatest diameter of the body of the spermogone, in 
P. murorum, is zth. The tissue of its body is hard, dense, whitish and hygro- 
metric: its cavity is generally simple. The spermatia are short, linear, straight 
or rod-shaped, with truncate ends, sometimes oblong or ellipsoid. Their length 
varies from oth to iz th, a large number measuring about ,,th to aath long. 
In many species they are almost atomic in size. Their breadth varies from wanth 
to scaooth. The sterigmata are normally, and as a general rule, articulated,—the 
component cellules or articulations being short, roundish or cubical, as in many 
Physcie. They are longish, varying from jth to ;,th, many of them being about 
math long, with a breadth of about ;,,th. In some species, as P. circinatum and 
P. fulgens, elongated, ramose, sterile, hypertrophied filaments, not unfrequently 
occur, growing from among the spermatiferous sterigmata, and filling the cavity 
of the spermogone. In exceptional cases, as in forms of P. circinatum and P. al- 
phoplacum, I have met with sub-simple sterigmata, which, however, is altogether 
an exceptional phenomenon in this genus. The sterigmata become, in progress 
of growth, almost solid, from thickening deposits in the interior of their individual 
component cells. 

