THE AMERICAN 
MONTHLY 
MICROSCOPICAL JOURNAL. 
Vou. VIIL. FEBRUARY, 1887. No. 2. 
On the Schwendener theory of the constitution of Lichens. 
By Fred’k Leroy Sargent. 
If we may judge of the importance of a subject by the amount of its liter- 
ature, it cannot be doubted that one of the most important questions that have 
been raised by the application of the microscope to the study of lichens is in 
regard to the origin and significance of their so-called gonidia. These are 
well known as the green cells which form a constant and conspicuous element 
of the lichen thallus, and to understand them is to understand what a lichen 
really is. The various hypotheses which have been offered to account for the 
existence of these gonidia accept as fundamental either one or the other of the 
following propositions. Either it is believed that these structures, green with 
chlorophyll, arise from the chlorophylless part of the lichen, and so may be 
considered as organs of the plant, or it is claimed that the gonidia are alge 
living among the tissues of a fungus and giving nutriment to it as host to 
parasite. In the first case a lichen is looked on as an autonomous plant; on 
the second view it is considered to be an association of two widely different 
kinds of plants living intimately together, and forming as it were a dual organ- 
ism. It will be most convenient for us, therefore, to speak of those holding 
the above views as either awtonomists or dualists respectively. 
It is the object of this article to deal more especially with the doctrine of 
the dualists—that doctrine first brought to the attention of lichenologists 
through the researches of the illustrious Schwendener, and which now is 
most widely known as the Schwendener theory. It is not proposed to give 
here all the proofs which have been offered in support of this theory, nor on 
the other hand shall we attempt to answer all the objections which have been 
made against it ; but we shall confine ourselves chiefly to a description of what 
we believe to be the views most generally held to-day by the followers of 
Schwendener, and to a brief statement of the more important facts on which 
such views are based. 
As we have just intimated, all lichens consist of a chlorophylless, fungus- 
like portion, and a chlorophyll-bearing alga-like part. Autonomists have 
claimed, however, that this resemblance of the lichen elements to fungi and 
algze respectively is merely a superficial one, and that there are, indeed, pro- 
found differences which can be demonstrated. Thus they say that not only 
do most lichens differ widely in the form of their so-called fungal part from 
any known fungi, but there are many lichen-gonidia unlike any alge that have 
been described. It may be freely admitted that the fungal and algal parts 
of many lichens, especially the more highly developed ones, differ consider- 
ably from any free-living fungi or alge ; but it must be urged that the differ- 
ences are exactly such as would be expected on the supposition that we have 
two kinds of plants living together under conditions more or less different from 
that of their non-lichenose relatives. For, while it must be admitted that the 
lichen-elements differ in many cases from the alge or fungi to which they are 
