Vll Rev. P. Keith on the Structure of Living Fabrics. 



of a dark and dull purple. Neither are they always furnished 

 with transverse or lateral nerves. These are proper to Dico- 

 tyledonous plants only, for in Monocotyledonous plants the 

 nerves are ail parallel. The point by which the leaf is at- 

 tached to the plant, is the base ; the opposite and terminating 

 point is the apex ; the intermediate body of the leaf is the ex- 

 pansion, and the boundary of the leaf is the margin. It often 

 happens that the base of the leaf is prolonged into a sort of 

 semi-cylindrical pedicle, by which the expansion is removed to 

 some distance from the point of attachment, as in the Vine and 

 Poplar. This pedicle is denominated the footstalk or petiole, 

 entering the expansion generally by the margin, but some- 

 times also by the centre, as in Nasturtium. In Populus fre- 

 inula it is compressed in a line at right angles to the expan- 

 sion, which peculiarity some phytologists regard as the cause 

 of the leaf's mobility*. 



The figure of the leaf or expansion has been found to be 

 of great use to botanists in the discriminating of the several 

 species of a genus ; and hence they have spared no pains to 

 determine by observation and description its varieties of form. 

 Linnaeus enumerates more than a hundred f. If the expan- 

 sion is flat and membranaceous, the most frequent forms are the 

 circular, the oval, the oblong, the triangular ; if thick and suc- 

 culent, the most frequent forms are the cylindrical, the semi- 

 cylindrical, the sword-shaped, the compressed. The apex is 

 acute, or obtuse, or bitten, or truncated, as in the leaves of the 

 Tulip-tree. The margin is entire, or notched, or toothed, or 

 serrated. The expansion is entire, or cleft, or lobed ; yet the 

 figure of some leaves is altogether anomalous, and cannot be 

 brought under any of the foregoing divisions. The leaf of 

 Nepenthes distillatoria 9 which is itself lanceolate, terminates at 

 the summit in a sort of thread-shaped pedicle, supporting a 

 hollow and cylindrical, or rather pitcher-shaped appendage, 

 to which there is attached the curious and peculiar process of 

 a lid opening at one side. This appendage secretes a fluid 

 which is said to be very pleasant to the taste. The leaf of 

 Dio?ia?a Muscipula is furnished with a process issuing from the 

 apex, which has a slight resemblance to a steel trap with the 

 wings expanded. This singular appendage is so highly irri- 

 table, that if it is but touched with the point of any fine or 

 sharp instrument, or if an insect but alights upon it, the lobes 

 immediately collapse as if eager to seize their prey, and detain 

 the insect captive, so that it resembles a trap, to which it has 

 been compared, not only in form but in function. 



In their size, leaves exhibit as much variety as in tJbcitt 



• Cours de Phylol Seance, i. 24. f Phil. Bot., sect. 83. 



