306 TRANSACTIONS, NATURAL HISTORY SOCIETY OF GLASGOW. 



affording an attractive feature, serve to strengthen 

 the plant and prevent it overbalancing. 



The North American Pitcher-plant (Sarraceiria 

 purpurea) affords evidence in support of this theory. 

 While its pitchers lie almost on the ground, and 

 would inevitably crush one another, a provision 

 similar in nature to the wings in the Nrpenthaceae 

 runs up one side of the pitcher. 



When the Nepenthes is allowed to attain any con- 

 siderable height, the pitchers assume a form that is 

 much better suited to their more exposed position. 

 Those formed lower down on the plants have no 

 power of supporting themselves, while those higher 

 up take the form of climbers, the elongated midrib 

 invariably twisting itself in the form of a screw or 

 spiral, which enables it to lay hold of any support 

 within its reach. The shape is also much altered, 

 and the wings are dispensed with, a much stronger 

 form being now necessary. The midrib is so 

 strengthened as to enable it to maintain a perpen- 

 dicular position. This is very important, as its 

 existence depends on the retention of the moisture 

 derived from the atmosphere in the form of rain or 

 dew. In the tropics the moisture at night, which 

 is occasioned by condensation, saturates everything 

 so completely that in Africa water-melons grow 

 luxuriantly in the sandy wastes. This would be 

 impossible without the drenching they receive 

 during the night. 



The rim round the mouth of the pitcher is much 

 the same in both forms, and the formidable bristles 

 that are arranged round its edge, pointing down- 

 wards, seem to suggest that it serves to keep its 

 prisoners safely secured. It also prevents the 

 pitchers being emptied when swinging about with 

 the wind, for they are able to retain a considerable 

 quantity of their contents even when hanging over 

 at right angles. It is well known that by keeping 

 them half-full of water they last several months 



