PROTECTION AGAINST UNBIDDEN GUESTS. 233 



the time when the plant commences flowering. This must be emphasized because 

 the suggestion has been made that the stipular secretion serves to protect the 

 foliage indirectly fi-om the ravages of caterpillars, snails, and beetles. The remark- 

 able observation has been made upon several plants, for the most part ti'opical, that 

 they live symbiotically with certain small and very fierce ants. The plants afford 

 the ants lodging in special cavities and give them nourishment in the form of sugary 

 and albuminous secretions: the ants in return defend the foliage against the attacks 

 of leaf-eating animals. So soon as this " standing army " of ants detects the foe it 

 commences offensive operations, like the garrison of a fortress, and by biting and 

 squirting formic acid frightens the invader away. In this way is protected the 

 foliage of Acacia spadicigera and s^ihwrocephala, Cecropia peltata, C'lerodendron 

 fistulosum, Rosa Banksiw, and several other plants (known as Myrmecophilous 

 Plants) against the attacks of leaf-eating animals. At the conclusion of this chapter 

 opportunity will offer to describe how the flower-buds of several Composites are 

 similarly protected against herbivorous beetles. In the case of Impatiens tricornis, 

 however, the ants are no protection for the foliage; whilst the leaves are developing, 

 no honey is secreted and no ants are present, and later, when honey is present in 

 plenty, and the ants are licking it up, they pay no attention, even though the 

 adjacent leaf-blades be touched or injured. 



Next to the diversion of creeping animals by means of nectaries scattered 

 over the stem and foliage may be ranked several arrangements in which the 

 protection aflforded is of a similar indirect character. Some of these have a 

 remarkable resemblance to the devices often employed by gardeners to shield the 

 plants in their propagating-pits and nurseries from the ravages of snails, cater- 

 pillars, centipedes, earwigs, and other noxious insects. In order to pi-eserve a hot- 

 house-plant from the visits of these undesirable members of the Animal Kingdoni, 

 gardeners very frequently place the pot containing the plant in question upon 

 another low pot inverted in a shallow dish of water; thus the plant stands, as it 

 were, on an island, and is inaccessible to the various creeping animals indicated. 

 Similarly in a nursery the crowns of the young trees are protected against creeping 

 vermin by tying a sticky cloth round the stem or painting the bark with bird- 

 lime or other sticky substance. Insects attempting to climb a tree under these 

 circumstances become imprisoned in the girdle. Caterpillars, snails, and other 

 animals with soft integument are often excluded by attaching belts of prickly 

 branches to the stems. 



When these expedients of the gardener are compared with many of the 

 arrangements met with in nature for the protection especially of the honey and 

 pollen, a remarkable similarity is at once obvious. Isolation by water, prevention 

 of access by means of sticky secretions, rings and fringes of prickles and thorns 

 directed so as to oppose visitors on foot — such, for the most part, are the methods 

 employed by plants to .secure immunity from would-be pilferers of their honey 

 and pollen. 



Isolation by water obtains in the case of innumei'able aquatic and bog- 



