446 BIOLOGIC RELATIONS BETWEEN PLANTS AND ANTS. 



their abode in swollen organs that act as water reservoirs, and conse- 

 quently aid the plant against drought, which may become suddenly 

 serious for any epiphyte. 



The lodging organs of these plants appear to be purely of physio- 

 logical origin. In other types they appear to have had primitively a 

 parasitic origin. We have already spoken of the analogy between the 

 tubercles of the myrmecophilous Pubiacepe and certain galls. But it 

 is more than doubtful whether the origin of these tubercles was primi- 

 tively parasitic and traumatic. In the types of which we have yet to 

 speak, parasitism, of animal origin, seems to have played an important 

 part, even in certain cases a primordial one, in the formation of myr- 

 mecophilous organs. 



We will first fix our attention on the myrmecophilous features of 

 Dischidia. 



The Dischidias are Asclepiadacese of the farthest Orient. With flex- 

 ible stems and branches they twine upon trees, and are especially noted 

 for possessing appendages in the form of pitchers. These are gener- 

 ally pendent from the branches, and into them plunge adventitious 

 roots that spring from the supporting peduncle (PI. XXI). The resem- 

 blance of these pitchers to the galls produced on the leaves of various 

 trees by aphides of the genus Pemphigus is such that a number of the 

 early observers of these plants considered them as abnormal organs 

 caused by the punctures of parasitic insects. 



The morphology of these curious organs has been fully elucidated by 

 the researches of Treub. They are modified leaves. The normal leaves 

 of Dischidia are orbicular, thick, fleshy, and opposite. A pitcher is 

 merely the blade of a leaf whose lower surface corresponds to the 

 inner surface of the pitcher, and whose petiole is thicker than that of 

 normal leaves. We can get a perfectly good idea of the formation of 

 these organs by imagining the blade of a normal leaf to be folded 

 toward the ground, then turned over and the borders brought together. 

 There is, besides, a change of growth in the young developing pitcher, 

 its increase being almost wholly along its middle, so that it takes the 

 form of a hood, with its opening first turned downward, then becoming 

 gradually set more or less upright. 



The Dischidias have opposite leaves, but the normal leaf opposite the 

 pitcher usually aborts. When the young urn takes on the form of an 

 elongated flask, there are produced upon its petiole some adventitious 

 rcots, of which those arising near the mouth of the pitcher enter its 

 cavity. A full-grown pitcher usually contains one or two long adven- 

 titious roots provided with a well developed system of radicles (fig. 

 1, PI. XXII). The internal surface of these pitchers is purple, while 

 their external surface is a grayish, glaucous green, like that of the sur- 

 face of the sterns and leaves. 



The direction assumed by the pitchers is variable and merits some 

 attention. The greater number are hung vertically with the mouth 



