420 BIOLOGIC RELATIONS BETWEEN PLANTS AND ANTS. 



The (Ecodomas are not the only ants that attempt the raising of 

 mushrooms. A Brazilian species of A tta digs subterranean passages 

 from its nest to the trees whose leaves it uses. These leaves, after 

 being taken to the nest, are torn up and masticated till they have the 

 appearance of a spongy gray mass. In this mass there develops the 

 mycelium of an agaric (Moller), which forms small white masses " like 

 cauliflower heads," the principal food of the ants. 



We have now referred to three kinds of ants that live at the expense 

 of plants, and whose depredations (if we disregard the benefit from the 

 dissemination of seeds by the harvesters and others) are injurious to 

 such plants. It is, therefore, not surprising to find in many vegetables 

 special defensive provisions made against the ravages of ants. It 

 should be noted that this provision is not always made against the 

 ants alone, but in a general way against injurious apterous insects. 



We have seen that certain harvesters attack the fruits of the Com- 

 positse. We find, accordingly, that certain typical forms in this family 

 surround their infl oresence with a regular chevaiix defrise. The exam pie 

 of the carline thistle shows this very clearly. The spinescent bracts of 

 the involucre form, as in many thistles, an insurmountable hedge. The 

 heads of the centaury have an involucre surrounded with little curved 

 needles, the rest of the plant being smooth. 



At certain times of the year a number of plants excrete from the 

 surfaces of their leaves a sugary liquid. This is the case with the oak, 

 whose leaves are in spring covered with ' honey dew." This excretion 

 is connected with the growth of the plant, and is caused, according to 

 various authors, by retarded transpiration. Primitively it must have 

 been a total loss to the vegetable ec'onomy, but little by little the plant 

 has become enabled to utilize it. Various stinging hymenoptera, such 

 as bees and ants, visit the leaves to gather this honey dew. There is 

 no doubt but that phytophagous animals can not approach a plant 

 thus covered with venomous insects without exposing themselves to 

 numerous stings. Hence there is, in exchange for food, a real protection 

 offered by the insects. 



In the case of the oak the production of the honey dew is not local- 

 ized, but extends over the whole surface of the leaf. But in the case of 

 some other plants the production is localized at certain special points 

 of the leaf which thus become true nectar glands, foliary or extra- 

 floral nectaries, also called extranuptial nectaries, since they in no 

 way contribute to the fertilization by insects. The production of the 

 honey dew at these specialized places is less abundant, but is more 

 constant than when it extends over the entire surface of the leaf. 

 Hence the nectarivorous insects are more constantly upon the surface 

 of the plant, and the protection against phytophagous creatures is 

 more efficacious. These extrafloral nectaries are placed upon the 

 aerial vegetative organs at points that vary in different plants. In the 

 cherry tree, for example, some nectaries, in the form of small, red 

 spherules, are found on the edges of the upper part of the petiole. 



