Leaves and their interaction with the environment  (Rust, fig. 43-44)

It is significant that plants are rooted in one spot, and are exposed in that spot to what­ever variation the environment presents. Such environmental variation must be tolerated for successful establishment, growth and reproduction. The leaf is the organ that experiences the greatest diversity in environmental conditions. Leaves must tolerate variation in of temperature, wind, sunlight, salt spray, moisture, and other environmental factors. Leaves lose water constantly; thus, they are often modi­fied in extremely dry, wet, or variably dry environ­ments. Chan­ges in leaf structure in most cases are compromises between the primary function of leaves, photosynthesis, and adaptive responses to environmental stresses. Leaves often show remarkable biophysical adaptations to the environ­ment, although more often selection has resulted in adaptations to stresses at the expense of adaptation to others. in this lab you will take a closer look at three factors that influence the way leaves interact with their environment, water, light, and temperature.

(A) Variation in moisture

Plants are sometimes classified as xerophytes, mesophytes, or hydrophytes based on their water relations. Xerophytes are adapted to dry environ­ments; mesophytes require available soil moisture and relatively humid air; hydrophytes generally grow completely or partially submerged in water. Struc­tural features of plants typical of such environments are referred to as xero­morphic, mesomorphic, and hydromorphic. When you are taught a "typical" plant structure such as a leaf, it is usually mesomorphic.

(1) Mesophytes

Prior to studying xeromorphic and hydromorphic leaves, review the structure of a mesophytic leaf, one that occurs in intermediate environments where moisture and light are not limited in the extreme. This should be familiar from your work in Biology 230.

(2) Xeromorphic Responses

Xeromorphic leaves are often small and thick, with a large palisade layer but reduced layer of spongy mesophyll, and a multi-layered epidermis. Such leaves may contain fibers for additional support, have thick cuticle layers, and stomata that are recessed in crypts. Some xeromorphic leaves are covered with hairs. How do each of these characteristics aid in the reduction of water loss? Succulent leaves are also common in plants of xeric environments; such leaves may also have some of the features described above.

(a) Mosses

Since these plants display little structural variation, how do they respond to lack of water? Obtain some of the dry bryophyte material available, and place it on the stage of a dissecting microscope. Note the condition of the "leaves". If a higher plant leaf were in this condition would it be able to recover? Focus on a high point of the material, and, while watching through the scope, apply a drop or two of water. What is the response? These plants have probably already begun photosynthesizing while you looked over to read this next sentence. Since there are no apparent structural adapta­tions, how is it that mosses may tolerate drought conditions? What physiological adjustments might be required?

(b)Nerium (oleander)

Nerium  is a xerophyte native to western Asia, and familiar to you as a shrub with pink or white blossoms planted in the center divider of freeways in the central valley. Obviously this is a habitat in which plants are subjected to periods of drought, not to mention other sorts of stress! Examine a portion of the leaf to one side of the midrib and find (a) the thick cuticle of the multilayered upper epidermis; (b) the multiple palisade parenchyma, with many crystals; and (c) bundle sheaths of parenchyma surrounding the veins. Portions of the lower epidermis are invaginated; these areas are known as stoma­tal crypts. Note that stomata and epidermal hairs are restricted to the crypts. Inasmuch as air currents increase water loss from the interior of leaves, how do crypts and hairs serve as adaptations to xeric environments?

(c)Pinus monophylla.

Piñon pine is a desert species from southwestern North America. This is an arid region by any measure. The needles have a low surface to volume ratio, typical of many xerophytes. Note the thickness of the cuticle and the fibers below the epidermis. Find the stomata. How would you describe their location? How would this be adaptive to xeric conditions? How loose is the mesophyll? Contrast looseness vs compactness of mesophyll. Which would be adaptive to xeric conditions, and why?

In fact, many conifers, especially pines, possess the above features; however, many conifers occur in northern latitudes and/or high elevations, where they experience freezing temperatures for much of the year. How do their xeromorphic features relate to cold environments?

(d)Yucca brevifolia.

Joshua trees occur in the Mojave desert. Their leaves are stiff, sharp-pointed leaves that grow in rosettes at the ends of branches. Examine a prepared slide of Yucca. At the same time make fresh sections of other succulents and compare them. First note the thickness of the cuticle. How does this thickness compare to other xerophytes? Why would extra thick cuticles be important to succulents? Find the stomata in the epidermis. Describe their location. Note the ridges of wax near the stomata. Is the location and the nature of wax near the stomata adaptive to xeric conditions? Examine the mesophyll. What does is mean to be succulent? Also note the conspicuous bundles of fibers. These fibers slough off from margins of yucca leaves, and have been used by humans in many ways. What do the fibers afford the plant?

Examine the other xerophytes on display, making a list of features that seem to be adapted to dry conditions.

(3) Hydromorphic Responses

Plants whose leaves are in constant contact with water do not experience water stress. On the other hand, there are other factors that may be limiting in aquatic environments, and plants of these area have a different array of leaf adaptations. Among these factors are oxygen availability, light, and flotation. Keep these in mind as you examine the hydrophytic leaves in the lab.

(a)Nymphaea (Castalia)

Waters lilies are aquatic, the leaves floating on the surface of water (the antithesis of a xerophyte). Examine a cross section. Locate stomata. On which surface do they occur? Is there a cuticle on the bottom of the leaf? How abundant are vascular tissues? What is the function of the large spaces in the lower epidermis?

(b) Additional Hydrophytes

Other hydrophytes are available for sectioning by hand. Examine sever­al of these. Contrast their structure with that of mesophytes or xerophytes. Note the nature of the epidermis and stomata, the occurrence of vascular tissue, and the degree of compactness of the tissues. What features do the hydrophytes seem to have in common?

(B) Variation in Temperature

The environment may exert conflicting demands on leaves. An important set of conflicts involves light‑gathering, temperature stress, and water loss. The primary function of leaves is to produce photosynthate on exposure to sunlight, but such exposure may cause leaves to gain heat, possibly subjecting them to stress. While evaporative water loss (through transpiration) can counterbalance this heat gain, water loss itself represents a potential stress that is mini­mized by the closure of stomata and other adaptations, as described above. Thus heat gain may be a significant problem in many plants, and we see several responses to this problem. Variations in leaf structure which may appear to be random are often adaptive biophysical adjustments in the morphology of the leaf. For a lobed leaf plant, a sun leaf may be small and deeply lobed, a shade leaf broad and barely lobed at all. While this doesn't seem particularly remar­kable, it actually represents a considerable adjustment to the temperature environments and the extremes these leaves are likely to encounter.

Spend a few minutes browsing through pages 38-59 of the Pacific Coast Tree Finder. Notice how most of the trees that grow in this relatively cooler, moist region have large, broad, simple leaves that are unlobed or shallowly lobed.

Now look at the Desert Tree Finder. In particular, examine the palos verdes on pages 17 and 25, the mesquites on pages 30 and 31, and the acacias on pages 26 and 29. Notice how small these leaves are compared to the leaves of Pacific coast trees, and how many are compound. Some desert trees even have the stem (rather than the leaves) as the main photosynthetic organ - for example the giant saguaro cactus).

            The same type of variation in leaf morphology can even be seen within the same species, and even on the same tree. For example, on a tree having lobed leaves, a "sun leaf" may be small and deeply lobed, while a "shade leaf" may be broader and minimally lobed.