20 BOTANY. 



of parenchyma ; the manner in which the vessels enter the leaf, and their 

 connexion Avith the stem, has already been referred to. Where the petiole 

 joins the stem, there is generally a constriction, and immediately external 

 to this, a swelling out, of cellular tissue. At other times the petiole is not 

 thus articulated, but either is a continuation of the stem or embraces it. 

 When articulated leaves drop, there is left a cicatrix or scar, which in 

 many cases is permanent. The petiole varies both in length and strength. 

 A compressed petiole, as in the Aspen, renders a leaf more sensitive to 

 slight currents of Avind. A phi/llodinm is a petiole compressed and 

 extended vertically, so as sometimes to supply the place of a leaf 

 Sometimes the petiole of a leaf runs out into a iendril or cirrhns ; more 

 frequently there is no blade whatever to such a petiole. 



A stipule is a membranous expansion or other process found on each side 

 of the base of a petiole. When attached to a leaflet, it is called a stipel. 

 Plants with stipules are stipulate ; without them, e.vstipulate. No definite 

 shape can be assigned to the stipule, its only characteristic being the 

 position above mentioned. 



Occasionally there arc anomalous forms of petiole and leaf, which merit 

 some special consideration. The true leaf is sometimes entirely absent, 

 and its place supplied by phyllodia or by stipules. Scales frequently replace 

 leaves, of which indeed they are to be considered as abortions. Several 

 leaves sometimes unite together, forming a connate leaf; when the basal 

 lobes of a leaf are united around the stem, it is perfoliate ; when the 

 laminae of a leaf run down and are united to the stem, it is decurrent. The 

 vascular bundles and parenchyma are sometimes separated or arranged so 

 as to inclose cavities, as in the tubular or fistular leaf of the onion, as also 

 in the ascidia or pitchers of such plants as Sarracenia or Nepenthes. 



Leaves occupy definite situations on the plant, and have special names in 

 difierent positions. When they arise from the crown of the root, they are 

 radical ; those on the stem are caidine ; on the branches, ramal ; on flower 

 stalks, floral. The leaves first developed are seminal; those appearing 

 subsequently, primordial. 



The arrangement of the leaves on the axis follows in definite order, and is 

 called phyllotaxis. Normally, the nodes from which the leaves spring are 

 ranged in a regular spiral round the stem. The internode between several 

 nodes may, however, be suppressed, so as to exhibit several nodes at the 

 same height on the stem. When two leaves are thus produced, at the same 

 level and on diff'erent sides of the stem, they are opposite ; when mgre than 

 two, verticillate or whorled. The imaginary line connecting the bases of 

 one pair of opposite leaves often crosses rectangularly the corresponding 

 line of the next pair ; the pairs are then said to decussate. 



When a lingle leaf is produced at a node, and each node is separated 

 from the next by an internode, the leaves are alternate. When, in a spiral 

 series of alternate leaves, one leaf is immediately below the third above it, 

 the arrangement is distichous : when it is the fourth which stands in this 

 relation, tristichous. In ftiis spiral arrangement, there are two elements, the 

 number of coils or turns of the spiral before one leaf is found to come 

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