lengthening of the elongated petiole causes the older leaves to float clear outside of the younger ones. In many biennial herbs, where food is stored up during the first season in preparation for the flowering effort in the second, a similar arrangement prevails--note the leaf-rosettes displayed by Spear Thistle (_Carduus lanceolatus_) and Herb Robert (_Geranium Robertianum_), as also especially in winter by perennials like the Dandelion (_Taraxacum officinale_) and Ribwort (_Plantago lanceolata_). Where stems spread horizontally, as the lower branches of trees, the leaves are arranged more or less in one [Illustration: FIG. 21.--AZARA MICROPHYLLA. 1/2.] 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 [Illustration: 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.”