28 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
THE SPORES. 
These bodies are usually eight in each ascus and rather small in 
size, but the number may vary from one to many and the size is 
inversely proportional to number, varying from 3 » to 200 # or more 
in length. (See pls. 3, 4, 9, 11, 18, 21, 22, 24, 32, 35, 40, 46.) The 
most common forms are oblong or ellipsoid, but the spores vary from 
acicular to spherical. Usually the size and form are quite constant 
in a given species, but there are instances of considerable variation 
even in the same hymenium. 
The spores as arranged in the ascus may be uniseriate, oblique, side 
by side, or more or less irregularly placed. They are likely to be side 
by side when acicular in form, uniseriate when spherical or oblong in a 
cylindrical ascus, and obliquely or irregularly arranged when the ascus 
is pyriform, thickly clavate, or ventricose-clavate. 
The spores may be simple, two-celled, four-celled, or several-celled, 
the cells being, in most of the compound conditions, arranged in a 
single series, separated by transverse walls. Besides the transverse 
divisions, others may be formed in the direction of the long axis of 
the spore, giving what is known as a muriform spore. In the devel- 
opment of the muriform spore the transverse walls always appear 
first, and in some species of lichens usually only a portion of the 
spores are ever found in the muriform condition, though the others 
are probably to be regarded as immature. The polar two-celled 
spore is a peculiar form found in Teloschistes and Placodium in 
which the two cells are far apart, one at either end of the spore. 
In color the spores vary from hyaline to a blackish brown. Simple 
spores are usually hyaline, but there are exceptions to the rule. Com- 
pound spores are very frequently more or less colored. 
The spores are not always easily distinguished in the ascus, espe- 
cially in rather thick sections. To bring them out better, the section 
may be carefully crushed on the slide, but one must always be sure that 
he sees the spores in the ascus, otherwise he may observe the spores 
of some other lichen and determine his specimen incorrectly. After 
making sure of the number of spores in each ascus and of their form 
and color, some may be studied outside of the ascus for the purpose 
of getting size, form, color, and structure more accurately. Minute 
spores may be mistaken in the ascus for the granular protoplasm of an 
immature ascus, and the acicular spores are also sometimes difficult 
to distinguish in the ascus. 
THECIAL ALGA. 
In some lichens there are found in the hymenium certain algal 
cells, commonly known as thecial or hymenial algw. These alge 
(fig. 6, p. 29) are usually smaller than those of the thallus, and are 
