672 LICHENS UNDER THE MICROSCOPE. 
late or globose. In some genera, as Calicium, they disappear 
early, and the spores then appear to be free. But they are 
usually persistent, and a little pressure is required to sep- 
arate the parts 
and bring out the 
spores. In the 
plant under ex- 
amination there 
are eight of them 
in each spore- 
case. This is the 
usual number. 
But many species 
have one, two, 
Section of Spermogonea of Theloschistes parietina. cl, cortical four, sixteen, or 
layer; g, gon o o, ostiolum; c, cavity; s, sterigmata; ml, 
me dullary laye 
more, or even 
several hundred spores in each spore-case. The spores differ 
greatly in size, form and color. In Theloschistes they 
arc colorless, of an oval form (Fig. 146), with a small 
cavity at each end, sometimes connected by a small canal, 
and measure from twelve to sixteen thou- Fig. 149. 
sandths of a millimetre in length. In other 
species they are of a brownish yellow, or a 
deep brown approaching black. The smallest 
spores are hardly two thousandths of a milli- 
metre in diameter, while the largest are 
nearly two-tenths of a millimetre in length. 
In form they are globose, oval, elliptical, fusi- 
form, needle-shaped, ete. (Fig. 147). Many 
spores are divided by one or more transverse 
partitions, and these again sometimes by per- 
pendieular ones. The former are called di- Sterigmata and sper- 
tetra-pleio-, or poly-blastish ; the latter mu- "a? rnesm* 
riform, and spores like those of Physica, polar-bilocular. 
Their great variety of form and eolor renders them most 
interesting objects under the microscope, and they are of 
