DEPENDENT PLANTS 48 
is but the reproductive part of it, and has little to do with 
the plant’s nourishment. 
Since the nutritive part of the plant is well distributed 
through the substratum and has abundance of nourishment 
at hand, the aerial structure can, at times, be produced with 
surprising rapidity. 
The economic importance of these destructive plants is 
readily seen when we consider that it is necessary to break 
up organized bodies and to return the substances composing 
them to a condition again fit for use by green plants. If 
these processes of decay brought about by saprophytes and 
parasites in both the plant and animal kingdoms did not re- 
duce materials in this way, we should eventually exhaust all 
the available food substances. 
Furthermore, when these organisms attack things which 
we desire to preserve for our own uses, the economic signifi- 
cance is of another kind. 
LESSON XXIII 
Mutualists 
Materials.—Specimens of several kinds of lichens illus- 
trating as wide range in habitat as possible; some specimens 
of clover, sweet clover, or alfalfa, on the roots of which there 
are tubercles. 
Lichens—habitat and general appearance.—Note where 
the lichens grow and the relation they bear to their support. 
Do they seem to be upon this support merely for benefit of 
position, or do they have parts penetrating the substance 
upon which they grow in order to obtain nourishment from 
them? 
Form.—Note the form of the lichen body. Is there much 
variation in form between different kinds? How does the 
* veindeer-moss ” lichen differ in form from those growing 
