SUMMARY OF OUR INTEREST IN PLANTS 375 
thrive in their former habitat, and possible for others to 
grow where before they could not. The replacing of one 
plant society by another is termed plant succession. When 
a forest is cleared and the tract burned over, the plant 
called fireweed appears in large numbers, even if a culti¬ 
vated crop is planted. After a year or two the fireweed 
gives way to a growth of 
blackberry and raspberry 
bushes, which are later 
replaced by grasses and 
weeds of various kinds. 
Another example of 
plant succession is seen 
in regions covered by 
fresh lava from a vol¬ 
cano. At first nothing 
grows. Probably bac¬ 
teria and fungi appear 
before other plants are 
noticed, but lichens are 
usually the first to be ob¬ 
served. These die and 
decompose, and their re¬ 
mains, together with bits 
of lava loosened by frost, 
wind, or water, accumu¬ 
late in depressions and 
form a soil in which 
mosses can grow. The 
remains of the mosses add to the organic matter in the 
slowly increasing soil, and, in the course of time, ferns and 
larger plants can grow. The last finally replace the mosses 
as they replaced the lichens. 
315. Summary of Our Interest in Plants. — Our first 
interest in plants is economic, that is, we think of them 
Sectional view of a branch infected 
with mistletoe, showing the relation be¬ 
tween the parasite and host; a, branch 
of host tree ; b, mistletoe ; c, primary 
sinker; d, sinker from cortical root; 
e , /, cortex of soft bark; g, cambium 
or growth ring; h, wood of branch. 
The starving and dwarfing of the branch 
beyond the mistletoe is shown at i. 
