42 Zoologica: N. Y. Zoological Society [HI; 3 



histological composition of the petiole, I here confine myself to a 

 general description, dwelling only on the more salient features. 



The petioles of the shade-plants are very long and slender 

 and their enlargement is fusiform and restricted to the base, 

 proximal to the leaflets. In cross-section (PI. IV) the enlarge- 

 ment is nearly circular, with one surface, corresponding to the 

 dorsal surface of the petiole, flattened. In the sun-plants the 

 basal thickening passes gradually into the leaflet-bearing portion 

 of the petiole and the cross-section (PI. Ill, figs. 3 and 4) is 

 distinctly triangular, one of the sides being dorsal, the two others 

 forming together the inverted roof-shaped ventral portion, so 

 that the petiole might be described as flattened dorsally and 

 carinate ventrally. The dorsal surface is distinctly winged, or 

 alate on each side. In the very young leaves the petiolar enlarge- 

 ment is solid, its interior being filled with juicy pith (PI. Ill, 

 fig. 3). This soon dries up, however, and is converted into a 

 flocculent or fibrillar, cinnamon brown substance, lying loosely 

 in a large cavity, the walls of which are lined by a thin layer 

 of the same reddish tissue. The same kind of tissue, the cells 

 of which are filled with a homogeneous amber-colored substance, 

 is also continually forming in four longitudinal strands or rays 

 in the dorsal wall of the petiole (PI. IV, fig. 1). Later a few 

 similar strands make their appearance also in the ventrolateral 

 walls. Since the amber-colored substance which characterizes 

 this tissue evidently has a high nutritive value, I shall call it the 

 nutritive parenchyma. In a few petioles in this stage I found 

 from four to six small Curculionid larvae feeding on the loo.se 

 material and reducing it to a red, powdery frass. Unfortu- 

 nately these larvae could not be reared, so that their specific iden- 

 tity is unknown. They are not, however, necessary agents in the 

 preparation of the petioles for their future occupants. 



As soon as the petiole has reached the stage just described 

 it is evident that any small insect, sufficiently enterprisng to bore 

 a hole in its wall, would find comfortable lodgings, which could 

 easily be rendered even more comfortable by tossing the dried 

 remnants of the pith out of the entrance or by compacting them 

 in the narrow ends of the spindle-shaped cavity. And if the 

 insect were a vegetable feeder it might find also an abundant 

 food-supply in the remains of the pith still forming the lining 



