FINK—THE LICHENS OF MINNESOTA. 15 
may be more or less immersed in the thallus, sometimes deeply, so 
that when the disk is more or less overgrown by the thallus or by a 
perithecium the structures are often quite obscured. The develop- 
ment of the apothecium begins below the surface of the thallus, and 
the tendency in general is to become more and more superficial as 
maturity is reached. Sometimes, however, it remains permanently 
more or less immersed, and somewhat varying conditions as to posi- 
tion with reference to the thallus may be expected in many species. 
STIPES AND PODETIA. 
These are structures which serve to raise the apothecium into the 
air, and are both to be regarded as originally developed for this pur- 
pose. In such genera as Calicium, Cyphelium, and Coniocybe this 
is their only function, and where this is the case they receive the 
name of stipes. But in Cladonia and Stereocaulon the stipe takes up, 
in addition, the office of fostering algal cells and by its vertical exten- 
sion and often ample branching greatly increases the area of surface 
behind which the alge may find protection. In this case the structure 
is called a podetium, the proper stipe being devoid of algal cells. The 
stipe, therefore, belongs to the fruiting tract, while the podetium, 
doubtless modified from this, has the essential character of the thallus 
and more properly belongs to the vegetative tract. 
RHIZOIDS AND CILIA. 
The rhizoids are found on the ventral side of most foliose thalli 
and serve as attaching organs. They appear to the eye as root-like 
bodies, varying in color from white to black. 
The cilia are like the rhizoids in structure, but are found on the 
upper surface of the thallus or along the margins. The hyphal 
rhizoids of the crustose lichens ¢ are quite different morphologically. 
The functions of cilia are doubtless to retain drops of water and 
gradually absorb them and sometimes, when quite numerous, to pro- 
tect against cold or dust. Closely related to rhizoids is the single 
attaching organ known as the wmbilicus on the ventral sides of the 
thalli of Gyrophoras, Umbilicarias, and many Dermatocarpons. 
OTHER STRUCTURES. 
Spermagonia, soredia, cephalodia, and cyphelle are structures 
which occur on or near the surface of thalli. The spermagonia (fig. 3, 
p. 16) are the supposed male reproductive organs, sometimes quite 
conspicuous as dark-colored spots on the upper surface of the thallus, 
asin some Parmelias and other large foliose lichens; but these struc- 
