plane, in such a manner that overlapping is reduced to a minimum ([Fig. 21]). This is well seen in horizontal branches of the Elm and other familiar trees. In the plant chosen for illustration (Azara microphylla, a Chilian shrub), an interesting arrangement obtains. One of the pair of stipules which subtends each leaf is itself leaf-like, and stands at an angle, so that a mosaic is formed of true leaves (the larger ones) and stipules (the smaller alternating ones). On all stems the leaves are arranged not at haphazard, but according to definite rules. Sometimes they
Fig. 22.—Leaf of Weinmannia trichosperma. 1/1.
are grouped in circles (whorls) at certain points of the stem, as in the Bedstraws; often in opposite pairs, arranged criss-cross, as in the Sycamore; most frequently in a series of spirals. The result in all cases is the same—it allows of as great an interval as possible between any leaf and the one immediately below or above it, and gives to all an equal share of light. The indenting of leaves, as in the Sycamore, or their division into separate segments, as in the Ash and Horse Chestnut, is of undoubted advantage as allowing light to pass through to lower layers of leaves; it also materially diminishes the danger arising from excessive wind-pressure. In the former case there is often a wide space between the divisions of the leaf; but where this is not required, the parts of the leaf fit closely together, to secure a maximum of surface. A particularly pretty example is seen in the Chilian shrub Weinmannia trichosperma ([Fig. 22]). Here, to avoid the loss of the area between the leaflets, the mid rib steps in, developing triangular wings which fill the spaces. It might be objected that the plant might have saved itself much trouble by producing, while it was about it, a simple undivided leaf covering the whole area. It is difficult to answer such suggestions. Probably the present form of the leaf best meets the conditions of wind, rain, and light under which it lives. Possibly its present form is bound up with its ancestral history. “It must be acknowledged,” says D. H. Scott, “that nothing is more difficult than to find out why one plant equips itself for the struggle with one device and another attains the same end in quite a different way.”
During cold and tempestuous weather the presence of leaves may be a danger to the plant rather than a help; and where seasonal variations are such that strongly contrasted periods of favourable and unfavourable weather occur, such as the summer and winter of our own climate, many plants have adopted the device of shedding all their leaves: this is especially characteristic of the largest plants (the trees), which would naturally suffer most from unfavourable weather. The fall of the leaf is accomplished by means of the formation of a transverse layer of corky tissue across the base of the leaf-stalk, combined with a weakening of the layer of cells immediately above. Prior to the perfecting of these arrangements for dropping the leaf, all the useful materials in it are withdrawn down the stem, so that only an empty skeleton is shed; the scar that remains is not an open wound, but is well protected by the corky layer before mentioned.
Stipules and bracts need not delay us in this sketchy survey of plant organs. They are leaves, generally of rather small size, placed, the former one on either side of the point where a leaf-stalk emerges from the stem, the latter singly below a flower; they are present in some plants, absent from others. They function in the same way as ordinary leaves, and in the earlier stages of growth are of use protectively. Occasionally the stipules exceed or even replace the leaves, as in the native Lathyrus Aphaca, where the leaf is reduced to a tendril, and the pairs of broad “leaves” are really the stipules. The bracts, in their turn, sometimes take on the “advertisement” function of the petals, as we have already seen ([p. 87]) in the case of certain Euphorbias.
The leaves of water plants offer several points of interest. Where they are entirely submerged, and, protected against the drying influence of wind and sun, they are of filmy texture. Broad blades are seldom met with, the leaves being usually either finely dissected or strap-shaped. The floating leaf, on the contrary, as already described in the Water Lily, is strongly built up, to withstand wave action and rain; it is usually broad and entire, which simplifies the
