Delphinidin

If the leaves of all plants could function normally under any environmental condition, various leaf modifications would provide no special benefits to a plant. But the form and structure of tropical rain-forest plants do not adapt them to thrive in a desert, and cacti soon die if planted in a creek because their structure, form, and life cycles are attuned to specific combinations of environmental factors, such as temperature, humidity, light, water, and soil conditions. The modifications of leaves occupying any single ecological niche may be very diverse, resulting in such a rich variety of leaf forms and specializations throughout the Plant Kingdom that only a few may be mentioned here.

- Shade Leaves
A single tree may have leaves that superficially all appear similar, but close inspection may reveal various differences. For example, because leaves in the shade receive less total light needed for photosynthesis, they tend to be larger than their counterparts in the sun. In addition, since they receive less intense sunlight and heat, they are thinner, and have fewer well-defined mesophyll layers and fewer chloroplasts. They also do not have as many hairs.

- Leaves of Arid Regions
Leaf modifications are generally more pronounced in different climatic zones or habitats. Because of the limited availability of water, wide temperature ranges, and high light intensities, plants growing in arid regions have developed adaptations that allow them to thrive under such conditions. Many have thick, leathery leaves and fewer stomata, or stomata that are sunken below the surface in special depressions, all of which reduce loss of water through transpiration.

They also may have succulent, water-retaining leaves or no leaves at all (with the stems taking over the function of photosynthesis), or they may have dense, hairy coverings. Pine trees, whose water supply may be severely restricted in the winter when the soil is frozen, have some leaf modifications similar to those of desert plants. The modifications include sunken stomata, a thick cuticle, and a layer of thick-walled cells (the hypodermis) beneath the epidermis.

The leaves of compass plants face east and west, with the blades perpendicular to the ground, so that when the sun is overhead, it strikes only the thin edge of the leaf, minimizing moisture loss.

- Leaves of Aquatic Areas
The submerged leaves of plants that grow in water usually have considerably less xylem than phloem, and the mesophyll, which is not differentiated into palisade and spongy layers, has large air spaces. Other modifications are described in the sections that follow.

- Tendrils
There are many plants whose leaves are partly or completely modified as tendrils. These modified leaves, when curled tightly around more rigid objects, help the plant in climbing or in supporting weak stems. The leaves of garden peas are compound, and the terminal leaflets are reduced to whiplike strands that, like all tendrils, are very sensitive to contact. If you lightly stroke a healthy tendril, there is a sudden, rapid growth of cells on the opposite side, and it starts curling in the direction of the contact within a minute or two. If the contact is very brief, the tendril reverses movement and straightens out again. If, however, the tendril encounters a suitable solid support (e.g., a twig), the stimulation is continuous, and the tendril coils tightly around the support as it grows.

Whole leaves of yellow vetchlings are modified as tendrils, and photosynthesis is carried on by the leaflike stipules at the bases. In the potato vine and the garden nasturtium, the petioles serve as tendrils, while in some greenbriers, stipules are modified as tendrils. In Clematis, the rachises of some of the compound leaves serve very effectively as tendrils.

Members of the Pumpkin Family (Cucurbitaceae), which includes squashes, melons, and cucumbers, produce tendrils that may be up to 3 decimeters (1 foot) long. As the tendrils develop, they become coiled like a spring. When contact with a support is made, the tip not only curls around it, but the direction of the coil reverses; sclerenchyma and collenchyma cells then develop in the vicinity of contact. The sclerenchyma cells provide rigid support, while the collenchyma cells impart flexibility. This makes a very strong but flexible attachment that protects the plant from damage during high winds. The tendrils of many other plants (e.g., grapes) are not modified leaves but develop instead from stems.

- Spines, Thorns, and Prickles
The leaves of many cacti and other desert plants are modified as spines. This reduction in leaf surface correspondingly reduces water loss from the plants, and the spines also tend to protect the plants from browsing animals. In such desert plants, photosynthesis, which would otherwise take place in leaves, occurs in the green stems. Most spines are modifications of the whole leaf, in which much of the normal leaf tissue is replaced with sclerenchyma, but in a number of woody plants (e.g., mesquite, black locust), it is the stipules at the bases of the leaves that are modified as short, paired spines.

Like grape and other tendrils, many spinelike objects arising in the axils of leaves of woody plants are modified stems rather than modified leaves. Such modifications should be referred to as thorns to distinguish them from true spines. The prickles of roses and raspberries, however, are neither leaves nor stems but are outgrowths from the epidermis or cortex.

- Storage Leaves
As previously mentioned, desert plants may have succulent leaves (i.e., leaves that are modified for water retention). The adaptations for water storage involve large, thin-walled parenchyma cells without chloroplasts to the interior of chlorenchyma tissue just beneath the epidermis. These nonphotosynthetic cells contain large vacuoles that can store relatively substantial amounts of water. If removed from the plant and set aside, the leaves will often retain much of the water for up to several months. Many plants with succulent leaves carry on a special form of photosynthesis called CAM photosynthesis.

The fleshy leaves of onion, lily, and other bulbs store large amounts of carbohydrates, which are used by the plant during rapid growth early in the subsequent growing season.

- Flower-Pot Leaves
Some leaves of Dischidia, an epiphyte (a plant that grows, usually non-parasitically, on other plants) from tropical Australasia, develop into urnlike pouches that become the home of ant colonies. The ants carry in soil and add nitrogenous wastes, while moisture collects in the leaves through condensation of the water vapor coming from the mesophyll through stomata. This creates a good growing medium for roots, which develop adventitiously from the same node as the leaf and grow down into the soil contained in the urnlike pouch. In other words, this extraordinary plant not only reproduces itself by conventional means but also, with the aid of ants, provides its own fertilized growing medium and flower pots and then produces special roots, which “exploit” the situation.

- Window Leaves
In the Kalahari desert of Botswana and South Africa, there are at least three plants belonging to the Carpetweed Family (Aizoaceae) that have unique adaptations to living in dry, sandy areas. Their leaves, which are shaped like ice-cream cones, are about 3.75 centimeters (1.5 inches) long and are buried in the sand; only the dimesized wide end of a leaf is exposed at the surface. This exposed end is covered with a relatively transparent, thick epidermis with few stomata and a waxy cuticle. There is a mass of tightly packed, transparent water-storage cells below the exposed end; these allow light coming through the “windows” to penetrate to the chloroplasts in the mesophyll, located all around the inside of the shell of the leaf. This arrangement, which keeps most of the plant buried and away from drying winds, allows the plant to thrive under circumstances that most other plants could not tolerate. Window leaves also occur in succulent plants of a few other families.

- Reproductive Leaves
Some of the leaves of the walking fern are most unusual in that they produce new plants at their tips. Occasionally, three generations of plants may be found linked together. The succulent leaves of air plants have little notches along the leaf margins in which tiny plantlets are produced, complete with roots and leaves, even after a leaf has been removed from the parent plant. Each of the plantlets can develop into a mature plant if given the opportunity to do so.

- Floral Leaves (Bracts)
Specialized leaves known as bracts are found at the bases of flowers or flower stalks. In the Christmas flower (poinsettia), the flowers themselves have no petals, but the brightly colored floral bracts that surround the small flowers function like petals in attracting pollinators. In dogwoods and a few other plants, the tiny flowers in their buttonlike clusters do have inconspicuous petals. However, the large white-to-pink bracts that surround the flower clusters, which appear to the casual observer to be petals, are  ctually modified leaves. In Clary’s annual sage (Salvia viridis), large colorful bracts are produced at the top of flowering stalks, well above the flowers.

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