Delphinidin

The earliest records of glass being used by humans date back to about 2600 B.C., when the ancient Egyptians and Babylonians made beads from the material. The use of glass panes for windows, however, did not begin until the Roman Imperial period a little over 2,000 years ago. Since then, the use of glass windows for admitting light to buildings of all sizes and shapes has become almost universal.

A comparatively recent use of glass involves solar energy as an alternative to nonrenewable sources of energy such as fossil fuels. The construction industry, particularly in the southwestern United States, is building new houses with flat panels and windows inclined at angles that maximize the amount of energy captured from the sun’s rays. Some buildings have mechanical devices that slowly move the solar panels so that they will follow the sun in its daily course across the sky. The use of such means of capturing solar energy is now spreading to other countries and could soon become commonplace.

Plants had a highly efficient form of solar panel that captured the sun’s energy many aeons before modern civilization began to realize that fossil-fuel supplies eventually would be exhausted. These remarkably constructed solar panels are the plant organs known to us as leaves. Regardless of their ultimate size or form, all leaves originate as primordia in the buds. In early spring, a leaf primordium may consist of fewer than 200 cells, but in response to changes in temperature, daylength, and availability of water, hormones are produced that stimulate these cells to begin dividing. Within a few days or weeks, the original 200 cells have multiplied, differentiated, and expanded into a structure consisting of millions of cells. In some plants (e.g., Eucalyptus), the first leaves produced (juvenile leaves) may appear quite different in form from those produced later. The juvenile form may be promoted by a class of hormones known as gibberellins.

At maturity, most leaves have a stalk, called the petiole, and a flattened blade, or lamina, which has a network of veins (vascular bundles). A pair of leaflike, scalelike, or thornlike appendages, called stipules, are sometimes present at the base of the petiole. Occasionally, leaves may lack petioles; when they do, they are said to be sessile. Leaves of deciduous trees normally live through only one growing season, and even those of evergreen trees rarely function for more than 2 to 7 years.

Leaves of flowering plants are associated with leaf gaps , and all have an axillary bud at the base. Leaves may be simple or compound. A simple leaf has a single blade, while the blade of a compound leaf is divided in various ways into leaflets. Regardless of the number of leaflets, a compound leaf still has a single axillary bud at its base, with the leaflets having no such buds.

Pinnately compound leaves have the leaflets in pairs along an extension of the petiole called a rachis, while palmately compound leaves have all the leaflets attached at the same point at the end of the petiole. Sometimes, the leaflets of a pinnately compound leaf may be subdivided into still smaller leaflets, forming a bipinnately compound leaf.

The flattened surfaces of leaves, which are completely covered with a transparent protective layer of cells, the epidermis, admit light to all parts of the interior. Many leaves twist daily on their petioles so that their upper surfaces are inclined at right angles to the sun’s rays throughout daylight hours.

Green leaves capture the light energy available to them by means of the most important process for life on earth, at least life as we know it today. This process, called photosynthesis, involves the trapping and ultimate storing of energy in sugar molecules that are constructed from ordinary water and from carbon dioxide present in the atmosphere. All the energy needs of living organisms ultimately depend on photosynthesis, from the first day of their existence to the last.

The lower surfaces of leaves (and in some plants, the upper surfaces as well) are dotted with tiny pores (stomata), which not only allow entry for the carbon dioxide gas needed for photosynthesis, but also play a role in the diffusion out of the leaf of oxygen produced during photosynthesis. Water vapor evaporating from the moist interior cell surfaces can also escape via the stomata. The evaporation of water can bring about some cooling of the leaf, but excessive water loss can result in damage to the plant. The stomatal apparatus, which consists of a pore bordered by a pair of sausageshaped guard cells, controls the water loss when the guard cells inflate or deflate, opening or closing the pore.

Leaves also perform other functions. For example, all living cells respire, and in the process of this and other metabolic activities, waste products are produced. These wastes accumulate in the leaves and are disposed of when the leaves are shed, mostly in the fall. Before dropping from the plant, the leaves are sealed off at the bases of their petioles. The following season, the discarded leaves are replaced with new ones.

Leaves play a major role in the movement of water absorbed by roots and transported throughout the plant. Most of the water reaching the leaves evaporates in vapor form into the atmosphere by a process known as transpiration . In some plants, there are special openings called hydathodes at the tips of leaf veins. Root pressure forces liquid water out of hydathodes, usually at night when transpiration is not occurring. The loss of water through hydathodes is called guttation. The expelled water may contain ions secreted by root cells.