24 PRACTICAL COURSE IN BOTANY 
adhesive weed pests, are no doubt very unwilling carriers of 
those disagreeable burdens. 
21. Tempting the appetite. — This is the most important 
adaptation to dispersal by animals. Have you ever asked 
yourself how it could profit a plant to tempt birds and beasts 
to devour its fruit, as so many of the bright berries we find in 
the autumn woods seem to do? To answer this question, 
examine the edible fruits of your neighborhood and you will 
find that almost without exception the seeds are hard and 
bony, and either too 
small to be destroyed 
by chewing, and thus 
capable of passing 
uninjured through 
the digestive system 
of an animal; or, if 
too large to be swal- 
lowed whole, com- 
pelling the animal, 
by their hardness or 
disagreeable flavor, 
to reject them. In 
cases where the seeds 
themselves: are ed- 
ible and attractive, 
the fruits are usually 
Fras. 40-42.—Adhesive fruits : 40, fruit ofhound’s- armed during the 
tongue ; 41, akene of bur marigold; 42, fruit of bur : . 
grass (cenchrus). growing season with 
protective coverings, 
like the bur of the chestnut and the astringent hulls of the hick- 
ory nut and walnut. The acidity or other disagreeable quali- 
ties of most unripe fruits serves a similar purpose, while their 
green color, by making them inconspicuous among the foliage 
leaves, tends still further to insure them against molestation. 
22. Voluntary agency. — The cultivated fruits and grains 
owe their distribution and survival almost entirely to the 
