20. BOTANY. 
of parenchyma; the manner in which the vessels enter the leaf, and their 
connexion with 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 
shght currents of wind. A phyllodium 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 tendril or cirrhus ; more 
frequently there is no blade whatever to such a petiole. Ge 
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 1s called a stipel. 
Plants with stipules are stipulate ; without them, exrstipulate. No definite 
shape can be assigned to the stipule, its only characteristic being the 
position above mentioned. 
Occasionally there are 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 
lamineze 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 m 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 
different positions. When they arise from the crown of the root, they are 
radical ; those on the stem are cauline ; 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 phyllotaris. 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 different sides of the stem, they are opposite ; when more than 
two, verticillate or whorled. The imaginary lime 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 this 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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