32 
1I0HENS. 
illustrations of this statement may be given Glceocapsidium and 
Protococcophila in the genus Lecidea; Emprostea and Peltidea of 
the genus Peltigera. 
It is found in consequence that in certain groups the gonidia 
afford a distinctive character of primary importance, accompanied 
by other characters of great weight. In other instances the 
differences of the gonidia coincide with specific differences, or 
constitute unimportant groups that are not characterized, except 
by the gonidia. 
For the above and other reasons it is not considered safe to 
found a natural classification on exclusively biological and physio- 
logical characters. 
The disposition of gonidia. — The disposition of gonidia has been 
unanimously considered a character of great importance. On this 
feature Flotow divides Lichens into two series, Lichenes 
Heteromerici and Lichenes Homceomerici , which corresponds in its 
principal features to the families Lichens and Byssacece of E. Fries, 
and to the Collemacece and Lichenacece of Nylander. A minute 
examination showed that, as a general rule, gonidia are only 
present in certain parts of the medullary layer of the thallus, where 
they receive sufficient light for their development. Certain parts 
of Lichens, especially the cortical layer and the hypothallus (also 
the marginal portions), are for some yet unexplained reason almost 
destitute of gonidia, and when exceptions occur, as the presence of 
gonidia in the cortical layer, as in Peltigera apthosa, they often 
cause the deformations known as cephalodia. 
The hymenium rarely contains gonidia, yet in a small number of 
Pyrenolichens and in the group Gonothecium (Wainio) of the genus 
Lecidea hymenial gonidia are present, and always differ in appear- 
ance from the gonidia of the thallus, and probably belong to a 
distinct species. 
In most genera of the higher Lichens the presence of gonidia is 
constant in the excipulum, consequently this character is unani- 
mously considered as one of great importance in the discrimination 
of genera and tribes. In Stereocanlon , Placodium , Pyxine , Stictis , 
Gyalecta, and numerous genera belonging to Pyrenolichens , the 
presence of gonidia in the apothecia is not always constant, and 
even when so their presence does not coincide with other more im- 
portant characters ; consequently they can only serve as characters 
for secondary groups. In Gyalecta perminuta (Wainio), in the 
same specimen, gonidia have been seen in certain apothecia, whereas 
in others they are entirely absent. 
The gonidia present in the excipulum are always, by one method 
or another, an extension of the gonidia existing in the thallus. 
(To he continued.) 
