The leaves of the thimbleberry at the edge of the trail were just starting to open. Unfold, actually. The youngest emerging leaves were tightly creased along each vein, with several layers of folds that held each leaf in a tight, organized crumple in the bud.
Slightly older leaves were already well open, showing their characteristic palm-shape, doubly-toothed edge, and fuzzy surface.
The increasing light and warmth signals the plant to start work for the season. The business of leaves is conducted in the middle layer of leaf tissue, where packets of chlorophyll manage the photosynthesis that produce the plant’s food. Microscopic pores (called stomata) on the underside of the leaf allow carbon dioxide in and let oxygen and water vapor out. As the vapor exits, water is drawn through the entire plant, driving its circulatory system. For most plants, the leaves are a sort of industrial district.
Although plants are grouped and definitively identified by their flowers, details of leaf structure are usually key to casual identification and to identifying plants not currently in bloom.
Though this plant didn’t have open flowers yet, the size of the shrub and the shape, color, and rather delicate spines of the twigs told me it was one of the berry members of the rose family. I knew it to be a thimbleberry because it had single leaf on each petiole (leaf stalk); the similar-looking salmonberry has three leaflets on each petiole.
While the diversity of leaf shapes and details may be very familiar, it’s worth looking at again. Note, for example, if the outer end of the leaf is rounded or flat, or some variation of pointed. Are the leaf margins flat, wavy, or rolled? Do the leaf margins have large or small teeth? Are the teeth pointed or rounded? Or are the teeth compounded, with smaller teeth and larger ones -- like the thimbleberry’s? Are the indentations deep enough to be lobes?
Look at leaf veins, too. Do the primary veins gather to a single point (as in thimbleberry)? Or do primary leaf veins run perpendicular from the center vein, or parallel to it, or some combination? How are the smaller leaf veins arranged -- branched or more lined-up?
Some plants have notable petioles or petiole structures. Cherry trees are easily identified by the tiny lumps on each leaf petiole, for example. These infant thimbleberry leaves already presented ultra-fine spines on the underside of the petioles and veins.
Sometimes the details of leaf structure also reveal specific adaptations that give clues to the plant’s habitat. For example, it’s not surprising that many plants in our dry-summer/wet-winter climate have leaves with waxy top surfaces: a waxy top helps retain water inside the leaf while allowing water to roll off before mildew sets in.
The fuzzy surface of thimbleberry leaves may help deter insect pests.
Dry environments also favor plants with thick, water-storing leaves or leaves with fuzzy undersides. The hairy undersides of some of our native plants, such as hairy manzanita, help to control water loss through the stoma.
The sunshine that will come sooner or later will electrify our plants’ emerging factories, illuminating the leaves’ lively greens and delicate details, and making leaf-watching especially rewarding. For deciduous perennial plants, such as the thimbleberry, these complex, varied, and vital organs are disposable and will be cast off by the plant by winter.
On the ground, these once-vibrant factories will be dismembered and eaten by the smallest animals and fungi. Thus broken down, the resources stored in the decayed leaves will be recycled -- much of it to be picked up by the same plant to be used to make another season’s leaves.