THE PETIOLE. 



51 



THE PETIOLE. 



285. THE LEAF-STALK, or PETIOLE, is usually a 

 half-round, the upper side being flattened, with a groove 

 or channel running through it, aa in the Crowfoot, fig. 1, 

 Plate XVII. 



The Petiole is 



(1) STIPULATE, invested at the base with a pair of 

 small leaves, called stipules, as ill the Cherry, fig. 2 ; 



(2) ALATE (winged], when invested on each side 

 with a process like a small section of the leaf, as in the 

 Orange, fig. 3 ; 



(3) SHEATHING, in such plants as the Coriander and 

 Dill, fig. 4; 



(4) CLASPING or TUBULAR, when it embraces the 

 stem, as in the Plane-tree, fig. 13 ; 



(5) ROUND, as in the Large Mallow, fig. 6 ; and the 

 Leaf is 



(6) SESSILE, when it has no petiole, but is seated 

 directly on the branch or stem, as in the Evening Prim- 

 rose, fig. 5. 



CHAPTER XX. 



LEAF MODIFICATIONS. 



286. The leaf, in many cases, takes what may be 

 called an abnormal development, producing various 

 irregular forms. The chief of these are, Phyllodia, 

 Ascidia, or Pitchers, Stipules, and Bracts. 



287. By an irregular expansion of the Fibrous sys- 

 tem, and a suppression of the Cellular, the petiole dilates 

 into a rigid blade (248), which is generally traversed by 

 parallel veins ; while the proper blade of the leaf becomes 

 partially, and sometimes wholly, abortive. These 

 leaf-like bodies are called PHYLLODIA. They are properly 

 a modification of the Petiole, and present some remark- 

 able peculiarities. You will remember that the regular 

 position of a leaf is parallel with the plane of the horizon, 

 or with one surface presented to the sky, and the other 

 to the ground ; but Phyllodia uniformly assume a vertical 

 position, presenting their edges, instead of their surfaces, 

 to the earth and sky. They are distinguished from 

 leaves with a true blade by their being always parallel 

 veined and entire ; while the early leaves, and occasionally 

 the later ones, are compound and feather veined. Phyl- 

 lodia sometimes appear making an effort to return to this 

 type, for not unfrequently they bear on their apex a true 

 compound, net-veined blade. These Phyllodia constitute 



General subject Common shape, varieties. Leaf modifications, varieties, 

 how produced. What are they properly ? Describe Phyllodia. Where found 

 'n what plants f How presented to the light ? What effect has such foliage f 



the entire foliage of the Australian Acacias, one of which 

 is seen at fig. 7 ; and these, with the Myrtaceous tribes 

 of the same region, compose nearly two thirds of the 

 entire forests of New Holland. In the Australian 

 Myrtles, the blade of the leaf becomes vertical, by a twist 

 in the petiole. A forest composed of such vegetation 

 has a grim and rigid look, and the effect of light and 

 shade is spectral. 



288. ASCIDIA, or PITCHERS. These are also to be 

 ranked among unusual developments of the petiole. 

 They are formed by the cohering edges of a dilated and 

 infolding petiole, and appear to be constructed for the 

 purpose of containing water. Such is the common 

 Pitcher-plant of our wet meadows (Sarracenia), fig. 9. 

 In this the tubular portion represents the petiole, while 

 the mouth-like process at the summit is the abbreviated 

 blade. In the Pitcher-plant of the East (Nepenthes), 

 the structure is still more curious. The petiole is first 

 expanded into a Phyllodium, then contracted into a ten- 

 dril, when finally it expands in the pitcher. This is fur- 

 nished with a neatly fitting lid, which is articulated with 

 it, as if by a regular hinge, made to open and shut, as 

 may be seen in fig. 8. It is usually found nearly filled 

 with water, which it is supposed to secrete through the 

 base of the petiole, whose spiral ducts are very large and 

 numerous. This liquid, whether water or a secretion of 

 the plant, always contains great numbers of putrifying 

 insects. The only living inhabitant is a kind of shrimp, 

 which preys on the insects, and probably lays its eggs in 

 their bodies. The water serves the double purpose of 

 attracting the flies and maintaining the shrimp ; and, so 

 far as the plant is concerned in this very curious and 

 singular economy, it may be nourished by the gases 

 evolved by the dead flies. This is not improbable, 

 since carbon, the basis of the vegetable body, would be 

 profusely supplied by the carbonic acid gas evolved 

 by the decomposing bodies. The uniformity of the 

 occurrence involves both habit and design. Some other 

 plants, also, have the same economy, which is probably 

 to be referred to the same cause. Foremost among 

 these is 



289. VENUS' FLYTRAP (Dionced). This plant pre- 

 sents one of the most remarkable instances of abnormal 

 structure in the petiole. This organ, as in the other 

 cases, is a dilated leaf-like body, net-veined, and bearing 

 on its summit a somewhat reniform or obcordate appen- 

 dage, fringed with a border of stiff bristles. This latter 

 process represents the blade of the leaf. In fig. 1 1 , two 



Ascidia. Instances. Native plant Exotic form. What contain ? What 

 tenant ? Describe Dionoea. Describe leaf. 



