It is well-known to every intelligent observer that plants are menaced
by a host of enemies. Though the plant cannot take up the aggressive to
any extent, the weapons which it employs in its own defense are of an
exceedingly efficient nature. In their way they are quite as effective
as anything that animals employ in their battle for existence.
Among the commonest defenses of the plant are spines, thorns and
prickles. In the sloe (_Prunus spinosa_), for example, the spines are
modified branches; in gorse (_Ulex Europæus_) they are branches and
leaves; and in cacti the green parts are thickened stems and the spines
reduced leaves; while in holly (_Ilex aquifolium_) the prickly leaves
answer the purpose of spines. The stinging hairs of the nettle which
exude an irritating acid when touched are a familiar example of
protection against vegetarian animals.
The way in which seeds are protected by spines is well illustrated in
the case of the Sweet Chestnut. Here it would be a very knowing animal
that could open one of the cases before they split naturally with the
ripening of the seed.