24 THE AMERICAN MONTHLY [February, 
extremely minute, it is easily carried to a considerable distance by wind or 
water. When a soredium is borne to a place where it can grow, it starts life 
as a miniature thallus, possessing the essential elements of a lichen, and future 
enlargement to the size of the parent is a matter of simple growth. Repro- 
duction by soredia is a method very frequently used by lichens for their propa- 
gation, and indeed the species are not a few in which soredia seem to have 
almost entirely taken the place of spores. Some of our very commonest lichens 
are rarely seen in fruit, but they have usually a copious supply of soredia. Such 
facts as these, it seems to us, should clear away any difficulty in understanding 
how lichens may be dual organisms and still grow in profusion where no free 
alge are found. 
There remains for us to consider one phase of the theory of dualism, 
which, although it has perhaps not always been accepted by dualists, is, we 
believe, accepted by the majority to-day, and it is one which affords an answer 
to an objection that seems to underlie a large share of the opposition encoun- 
tered by the Schwendener theory. This objection is to the effect that, since 
there is a direct antagonism between a parasite and its host, the supposition 
that a lichen consists of a fungus parasitic on alge is untenable. For if the 
gonidia are algal hosts we should expect them to speedily succumb under the 
attacks of their parasite, and development of the lichen would be impossible. 
If, however, on the other hand, we view these gonidia as products of the 
hyphe, generated continually, their increase along with the development of 
the thallus becomes comprehensible. Let us view this objection in the light 
of physiology. Whether these gonidia are produced by the hyphe, or are 
algal hosts, there is but one opinion warrantable regarding their physiolog- 
ical relation to the rest of the lichen. They all possess chlorophyll, while 
in all other parts of the lichen this substance is absent. Now, there is per- 
fect agreement among vegetable physiologists in the belief that all cells pos- 
sessing chlorophyll are capable of utilizing the energy of sunlight to decom- 
pose carbon dioxide, and that then they combine the carbon with other inor- 
ganic substances to form such energized food as starch and other carbohy- 
drates. Cells destitute of chlorophyll require for their growth a supply of 
such energized food and soon die without it. Suppose we take now a lichen 
growing on the bare surface of a rock; all the food it can get is of an inor- 
ganic nature. The only parts of the lichen which can make use of such sub- 
stances are the gonidia, and if the hyphal part of the lichen is to live it must 
obtain energized food from these chlorophyll-bearing structures, and that is 
just what a parasite would do. 
It thus becomes apparent that if there is any force in the objection which 
supposes that a mortal antagonism necessarily exists between a parasite and 
its host, the autonomists have as grave a difficulty to face as the dualists. 
But the difficulty itself is, we believe, an entirely imaginary one, as we shall 
proceed to show in taking up that phase of dualism to which we have re- 
ferred above. 
We have already said that gonidia have been observed to grow more vig- 
orously after contact with hyphe. In this connection it must be added that 
the formation of soredia takes place as a result of such an unusually rapid 
multiplication of gonidia that at certain points the cortex of the thallus is 
ruptured. We would infer from such facts that the reverse of antagonism 
existed between the food-producing and the food-consuming parts of lichens. 
These considerations lead us to the question: Are there not some important 
benefits which terrestrial alge might gain by being associated with fungi in 
the manner which dualists claim takes place in lichens? 
First, let us see what are the conditions which favor the life-processes of an 
alga, A liberal supply of water, containing certain salts in solution, is im- 
