162 
PLANT STUDIES 
nary crops have exhausted the soil of its nitrogen-contain- 
ing salts, and it has become comparatively sterile, clover is 
able to grow by obtaining nitrogen from the air through the 
root-tubercles. If the crop of clover be " plowed under," 
nitrogen-containing materials which the clover has organ- 
ized will be contributed to the soil, which is thus restored 
to a condition which will support the ordinary crops again. 
This indicates the significance of a very ordinary " rotation 
of crops." 
(4) Ant-plants, etc. In symbiosis one of the symbionts 
may be an animal. Certain fresh-water polyps and sponges 
become green on account of Algae which they harbor with- 
in their bodies (Fig. 152). Like 
the Lichen -fungus, these ani- 
mals use the food manufactured 
by the Algae, which in turn find 
a congenial situation for living. 
By some this would also be re- 
garded as a case of helotism, 
the animal enslaving the alga. 
Very definite arrangements 
are made by certain plants for 
harboring ants, which in turn 
guard them against the attack 
of leaf-cutting insects and oth- 
er foes. These plants are called 
Myrmecopliytes, which means 
" ant-plants," or myrmecopMlous 
plants, which means "plants loving ants." These plants 
are mainly in the tropics, and in stem cavities, in hollow 
thorns, or elsewhere, they provide dwelling places for tribes 
of warlike ants (Fig. 153). In addition to these dwelling 
places they provide special kinds of food for the ants. 
(5) Flowers and insects. A very interesting and impor- 
tant case of symbiosis is that existing between flowers and 
insects. The flowers furnish food to the insects, and the 
FIG. 152. A fresh-water polyp (Hy- 
dra) attached to a twig and feed- 
ing upon algae (C), which may 
be seen through the transparent 
body wall (,B).^GOLDBERGER. 
