66 



LEAVES. 



pemrew* are brought in contact from the unusual development 

 of both systems of the leaf, and they grow together, forming 

 a perfoliate leaf, (fig. 57.) The same takes place in many 

 other plants, and the occurrence differs in no respect from 

 what happens in the production of twin apples, and other sim- 

 lar formations except in its uniformity, which DeCandoUe de- 

 nominated constant accidents. Why it should uniformly oc- 

 cur, and only in the upper leaves, we are unable to explain, by 

 any secondary cause with which we are acquainted ; but by ex- 

 amination 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 phe- 

 nomena. The other leaves are of the oval lanceolate form, with 

 the veins forming acute angles with the midrib, but in the perfo- 

 liate 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 oth- 

 erwise form a cordate leaf, as in the Uvula- 

 ria perfoliata, fig. 58. But the most singu- 

 lar variations produced by the operation of 

 this cause, occurs in the pitcher like leaves. 

 Our common Sarracena (Side-saddle flow- 

 er) is produced by the cohesion of the edges 

 of the leaf, or as is most generally sup- 

 posed of the petiole only, and the expan- 

 sion at the top of the cup is thought to be 

 the real lamina, which is probably the 

 case, (fig. 59.) The 

 Nepenthes or Pitcher 

 plant of India, pre- 

 sents a still more 

 striking instance of 

 variation, and partly 

 from the cause under 

 consideration This 

 singular leaf, ex- 

 hibited in fig. 60, 

 is described as aris- 

 ing" from the stem 

 with a round common petiole, like 

 most other leaves, which soon ex- 

 pands into a lamina and after- 

 wards becomes round, long and 

 slender, resembling a tendril. At 



