in the pea and bean family, in rosaceous plants and the family
Rubiaceae. They are not common in dicotyledons with opposite
leaves. Plants having stipules are called stipulate; those having
none are exstipulate. Stipules may be large or small, entire or divided,
deciduous or persistent. They are not usually of the same form as the
ordinary foliage leaves of the plant, from which they are distinguished
by their lateral position at the base of the petiole. In the pansy
(fig. 24) the true
leaves are stalked
and crenate, while
the stipules s are
large, sessile and pinnatifid.
In Lathyrus
Aphaca and some
other plants the true
pinnate leaves are
abortive, the petiole
forms a tendril, and
the stipules alone are
developed, performing
the office of
leaves. When stipulate
leaves are opposite
to each other,
at the same height
on the stem, it occasionally
happens
that the stipules on
the two sides unite
wholly or partially,
so as to form an
interpetiolary or
interfoliar stipule, as in
members of the
family Rubiaceae. In the case of alternate leaves, the stipules at
the base of each leaf are sometimes united to the petiole and to each
other, so as to form an adnate, adherent or petiolary stipule, as in the
rose, or an axillary stipule, as in Houttuynia cordata. In other
instances the stipules unite together on the side of the stem opposite
the leaf forming an ocrea, as in the dock family (fig. 25).
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Fig. 21.—Leaf of Orange (Citrus Aurantium), showing a winged leafy petiole p, which is articulated to the lamina l. |
Fig. 22.—Pitcher (ascidium) of a species of Side-saddle plant (Sarracenia purpurea). The pitcher is formed from the petiole, which is prolonged. |
In the development of the leaf the stipules frequently play a most
Fig. 23.—Leaf of an Acacia (Acacia
heterophylla), showing a flattened
leaf-like petiole p, called a phyllode,
with straight venation, and a bipinnate
lamina.
important part. They begin
to be formed after the origin
of the leaves, but grow much
more rapidly than the leaves,
and in this way they arch over
the young leaves and form
protective chambers wherein
the parts of the leaf may develop.
In the figs, magnolia
and pondweeds they are very
large and completely envelop
the young leaf-bud. The stipules
are sometimes so minute
as to be scarcely distinguishable
without the aid of a lens,
and so fugacious as to be
visible only in the very young
state of the leaf. They may
assume a hard and spiny
character, as in Robinia
Pseudacacia (fig. 19), or may be
cirrose, as in Smilax, where each
stipule is represented by a
tendril. At the base of the
leaflets of a compound leaf,
small stipules (stipels) are
occasionally produced.
Variations in the structure and forms of leaves and leafstalks are produced by the increased development of cellular tissue, by the abortion or degeneration of parts, by the multiplication or repetition of parts and by adhesion. When cellular tissue is developed to a great extent, leaves become succulent and occasionally Modifications. assume a crisp or curled appearance. Such changes take place naturally, but they are often increased by the art of the gardener, and the object of many horticultural operations is to increase the bulk and succulence of leaves. It is in this way that cabbages and savoys are rendered more delicate and nutritious. By a deficiency in development of parenchyma and an increase in the mechanical tissue, leaves are liable to become hardened and spinescent. The leaves of barberry and of some species of Astragalus, and the stipules of the false acacia (Robinia) are spiny. To the same cause is due the spiny margin of the holly-leaf. When two lobes at the base of a leaf are prolonged beyond the stem and unite (fig. 26), the leaf is perfoliate, the stem appearing to pass through it, as in Bupleurum perfoliatum and Chlora perfoliata; when two leaves unite by their bases they become connate (fig. 27), as in Lonicera Caprifolium; and when leaves adhere to the stem, forming a sort of winged or leafy appendage, they are decurrent, as in thistles. The formation of peltate leaves has been traced to the union of the lobes of a cleft leaf. In the leaf of the Victoria regia the transformation may be traced during germination. The first leaves produced by the young plant are linear, the second are sagittate and hastate, the third are rounded-cordate and the next are orbicular. The cleft indicating the union of the lobes remains in the large leaves. The parts of the leaf are frequently transformed into tendrils, with the view of enabling the plants to twine round others for support. In Leguminous plants (the pea tribe) the pinnae are frequently modified to form tendrils, as in Lathyrus Aphaca, in which the stipules perform the function of true leaves. In Flagellaria indica, Gloriosa superba and others, the midrib of the leaf ends in a tendril. In Smilax there are two stipulary tendrils.
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Fig. 24.—Leaf of Pansy. s, Stipules. |
Fig. 25.—Leaf of Polygonum, with part of stem. o, Ocrea. |
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Fig. 28.—Pitcher of a species of pitcher-plant (Nepenthes distillatoria). |
The vascular bundles and cellular tissue are sometimes developed in such a way as to form a circle, with a hollow in the centre, and thus give rise to what are called fistular or hollow leaves, as in the onion, and to ascidia or pitchers. Pitchers are formed either by petioles or by laminae, and they are composed of one or more leaves. In Sarracenia (fig. 22) and Heliamphora the pitcher is composed of the petiole of the leaf. In the pitcher plant, Nepenthes, the pitcher is a modification of the lamina, the petiole often plays the part of a tendril, while the leaf base is flat and leaf-like (fig. 28).
In Utricularia bladder-like sacs are formed by a modification of leaflets on the submerged leaves.
In some cases the leaves are reduced to mere scales—cataphyllary leaves; they are produced abundantly upon underground shoots. In parasites (Lathraea, Orobanche) and in plants growing on decaying vegetable matter (saprophytes), in which no chlorophyll is formed, these scales are the only leaves produced. In Pinus the only leaves produced on the main stem and the lateral shoots are scales, the acicular leaves of the tree growing from axillary shoots. In Cycas whorls of scales alternate with large pinnate leaves. In many plants, as already noticed, phyllodia or stipules perform the function of leaves. The production of leaf-buds from
