58 LEAVES. 



place in many other plants, and the occurrence differs in no 

 respect from what happens in the production of twin apples 

 and other similar formations, except in its uniformity, which 

 De Candolle denominated constant accidents. Why it should 

 uniformly occur, and only in the upper leaves, we are unable to 

 explain by any secondary cause with which we are acquainted ; 

 but by examination of the leaves, we are irresistibly led to the 

 conclusion, that the slight variation in the direction of the 

 veins and the great development of the parenchyma are the 

 causes of the phenomenon. The other leaves are of the oval 

 lanceolate form, with the veins forming acute angles with the 

 midrib ; but in the perfoliate leaves the veins pass off at nearly 

 right angles, with a much more abundant production of the 

 parenchyma, thus uniformly accomplishing in this case what 

 occasionally happens in other vegetables. Perfoliate leaves 

 occur, from the same cause, in the alternate varieties, by the 

 union of the lobes, of what would otherwise form a cordate 

 leaf, as in the Uvularia perfoliata. But the most singular 

 variations produced by the operation of this cause, occur in the 

 pitcher-like leaves. Our common Sarracenia (Side-saddle flower) 

 is produced by the cohesion of the edges of the leaf, or, as it is 

 most generally supposed, of the petiole only, and the expansion 

 at the top of the cup is thought to be the real lamina, which is 

 probably the case. The Nepenthes or Pitcher-plant of India, 

 presents a still more striking instance of variation, and partly 

 from the cause under consideration. This singular leaf, ex- 

 hibited in Fig. 92, rises from the stem with a round corn- 

 Fig. 92. 



Leaf of the pitcher-plaut. 



mon petiole, like most other leaves, which soon expands into 

 a lamina, and afterwards becomes round, long, and slender, 

 resembling a tendril. At the extremity of this tendril is de- 

 veloped the pitcher, with a lid closely fitting its orifice. The 

 whole of this curious production, except the lid, is supposed 



How in alternate leaves ? How is the leaf of the Sarracenia constructed ? 

 How the Pitcher-plan*? 



