SEED DISPERSAL 
241 
some one unfavorable condition might kill them all. In order 
to scatter seeds a plant makes use of wind, animals, and water 
as distributing agents, and of such mechanical devices as 
exploding pods. To be distributed by the wind, a seed or a 
fruit must be light. This is brought about in some plants 
by plumes as in the case of the akene of the dandelion, 
thistle, and clematis, by the down on a milkweed seed, and 
by the wings on the fruits of the maple and elm. (Review 
§ 209.) To be carried by 
animals, the adaptations 
may be of two kinds — 
either they must have 
hooks to catch on or they 
must be edible. The bur¬ 
dock and beggar’s tick 
represent the first kind 
of adaptation, and the 
fleshy fruits the second. 
In some cases only the 
fruit is eaten, as with 
peach, plum, etc., and 
the seeds dropped; and 
in other cases the seeds 
and all are eaten, but are 
not acted upon in the 
digestive tract, and so 
are dropped far from the spot where they grew, as with 
berries, apples, pears, etc. (Review § 212.) 
To be carried by water, the adaptations must be such 
as will insure buoyancy and the ability to withstand decay. 
Both are well illustrated by the coconut, and the former 
by the cocklebur. To be distributed by being forcibly 
expelled, the fruits must have elastic tissue. The jewel- 
weed or touch-me-not illustrates this, as does witch-hazel, 
wild cucumber ; and violet (see Figure 225). 
Figure 231. — Fruits Distributed by 
Animals. 
A, avens ; B, hound’s tongue ; C, cockle- 
bur; D, burdock. 
