80 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
tracted, by means of an anti-toxin extracted from the blood of 
the horse. 
Turning to the other side of the question, we find that 
numerous plants are protected by various means against vege- 
tarian animals. Many species, for example, more or less suc- 
cessfully ward off the attacks of browsing forms by mechanical 
devices. Of this nature are the thorns, spines, prickles, and 
stinging hairs with which painful experience has made most of 
us more or less familiar. Good illustrative cases are such com- 
mon forms as gorse, blackthorn, holly, thistle, and stinging-nettle. 
This is not, however, the only use of sharp-pointed outgrowths, 
for not a few forms, ¢.g. the bramble, are “hook-climbers”. It 
is particularly noticeable that ripening fruits are often mechani- 
cally guarded, the prickly husks of horse- 
chestnut being a case in point. 
Quite a number of herbaceous plants 
contain in their soft tissues bundles of 
_ exceedingly sharp needle-like crystals 
Fig. 1075.—Cell from Leaf of Vir- ; : 
ginia Creeper (Ampelopsis), containing (raphides, fig. 1075) which protect them 
a bundle of needle-crystals (raphides) : : 
of oxalate of lime; highly magnifed against the ravages of slugs and snails, 
as experiment has shown. Many such 
crystals are to be found, for example, in the leaf-stalks of the 
Arum-“ Lily” (Richardia 4 thiopica). 
Many plants are protected by chemical means, ze. by the 
formation within their tissues of substances which are poisonous 
or nauseous, or otherwise detrimental to the well-being of would- 
be consumers. Forms such as Foxglove, Aconite, Monkshood, 
Hemlock, and Yew no doubt ward off to a great extent the 
attacks of browsing animals in this way. Fruits and seeds 
are often thus protected, and may be either simply nauseous 
(especially when unripe) or else contain active poisons, as in the 
case of the seeds of Laburnum and Strychnos nux-vomica. The 
attacks of both large and small animals are checked in many 
cases by means of a sticky fluid known as J/atex, which flows 
from an injured part, and when fresh has a milky appearance. 
Common examples among British plants are the Spurges 
(Euphorbia), Poppy, Greater Celandine, and Dandelion. This 
secretion hardens when dry, and forms a protective coat over the 
wound. Some tropical trees produce a kind of latex which, in the 
solidified condition, is known to us as india-rubber. It is almost 
