Forest form and function as influenced by light: Reference:Trees structure and function by Zimmerman & Brown) Photos: E. Iglich on WMC campus Light, along with gravity, is one of the most important parameters of the environment in determining the course of development in woody and herbaceous plants. The growth and form of plants from the time of seed germination to maturity is directly affected by light intensity, quality and duration." I. Phototropism: Phototropism = effect of light on the direction of growth. Most likely you have all noted a plant in door 'reaching' for the light at the window. Phototropic responses of individual twigs play an important role in the positioning of branches in the much-branched crowns of current species. Consider a Brach which is dichotomously branching ( splits into 2 at tip and so on) - slight changes in angularity and length will create a major change in final shape. Light. gravity, and competition for growing space interact to determine the overall size and shape of tree crowns. Picture from text Forest Stand Dynamics by Oliver & Larson All aspects of growth are impacted by the direction, intensity and quality of light. ..Seedlings grown in low light will have : External modifications: Internally, such plants have: longer internodes; thinner stems; broader, thinner leaves; and poorly developed root systems. longer and thinner-walled parenchyma cells. reduced vascular tissues, and weakly lignified tracheary elements. How does this affect plant strength? ability to withstand storms? For each of the following trees on campus, explain whether they are exhibiting strong or weak epinasty, and whether they are examples of trees in full or partial exposure:

Leaves: Leaves, however, respond differently from stems and much significance has been attached to the physiological and structural qualities of shade versus sun leaves. Sun leaves are: Shade leaves are: more xerophytic in nature, possess thicker cuticles, more and longer palisade cells well developed vascular tissues. thinner and weakly lobed large surface area less support tissue fewer stomata Which of the following is the shade leaf? how can you tell?

• III. Influence of light on species distribution: Specific species are known to tolerate low light ( shade) conditions better than others. Trees in early successional stages are typically shade intolerant, while those especially climax species are shade tolerant. Shade Tolerance of American Forest Trees (Some trees are difficult to place in one category with assurance (some seem to behave differently in some regions of the country, for example).
  Tolerance Classes ( Bolded trees are those we can see locally) Very Intolerant ( must have full light r close to it : Most willows, Quaking aspen, Bigtooth aspen, Cottonwoods, Black locust, Osage orange Intolerant: accept some shade Black walnut Butternut Pecan Hickories Paper birch Yellow poplar ,Sassafras, Virginia pine Sweetgum, Sycamore, Black cherry, Honey locust, Kentucky coffeetree
  	Intermediate: Eastern white pine Yellow birch American chestnut, White oak, Red oak, Black oak, American elm, Rock elm, Hackberry ,White ash & Green ash, Black ash Tolerant: Red maple, Silver maple, Box elde,r Basswood, Tupelos, Persimmon, Buckeyes Very Tolerant: Eastern hophornbeam, American hornbeam, American beech, American holly, Sugar maple Flowering dogwood How can a plant be 'adapted' for a specific tolerance level? what physiological or morphological traits would it have? Which of the trees pictured on campus would expect to find in shady locations? sunny locations? Intermediate? the top right is a hickory, to the left a dogwood and to the bottom right an oak

III. Light quality, i.e. the various wave lengths of light, have been shown to effect these same morphogenetic processes quantitatively.

In general, it seems that : blue light hastens the cessation of cell division and elongation to a greater degree than red light, and red light is more effective than far-red; whereas, the greatest cell numbers and lengths are obtained in complete darkness. Explain why! (Theodoresco, 1929; Vince, 1956; LeNoir, 1967) .Offer an evolutionary explanation for this phenomenon. Plants growing in blue light of sufficient energy tend to develop in essentially the same way as those grown in white light, while those grown under red or far-red light tend to develop etiolated(spindly) characteristics. As light passes through the canopy preferential wavelengths are absorbed. These would be?? leaving the lower canopy species with which wavelengths? How would this influence their growth?  Reflection of wavelengths off a leaf is as follows: IR 70% reflected Visible 6-12% reflected ---> Red/orange: 3-10% vs. green 10-20% UV 3% reflected Total light reaching a forest floor is only 1-5% of entering canopy, thus encouraging shaded individuals to shoot up as soon as possible.

Photoperiod: Plants and animals Circadian rhythms are innate rhythms of activity and inactivity covering approximately 24 hrs and is characterized in all living organisms except bacteria. They effect physiological processes as well as metabolic. Light or temperature is the time setter to match the internal & external cycles Plants: The effect of the daily duration of light, or photoperiod, on growth and development in woody plants has been studied extensively in recent years. Because the length of day varies considerably from the tropics to the higher latitudes, it has had a significant adaptive influence on the growth habit of plants.

Although there are wide differences among woody plants in their reaction to photoperiod, even among species growing together at the same latitude, photoperiodism has been demonstrated to have a marked effect on many physiological processes involving growth and development. Among these are: rate and duration of shoot and diameter growth, breaking of dormancy, cessation of growth, seed germination, leaf abscission, frost resistance, flowering response.

In animals: Proteins May Control Body Clock....................By REUTERS...........LONDON -- Collaborating Dutch and Japanese scientists have isolated two proteins that could be the timers for the body's internal clock that controls sleep patterns, alertness and jet lag. Jan Hoeijmakers and researchers at Erasmus University in Rotterdam and a group of scientists at Tohoku University in Sendai said proteins called Cry1 and Cry2 control the tick-tock of the internal clock in the brains of mammals. The proteins seem to adjust the body's clock, also known as circadian rhythms, to changes in light during the different seasons and are an integral part of the clock itself. "Without these proteins there is no biological clock running. They may have at least two separate functions -- providing the light input into the clock and adjusting it to changing light conditions as well as being part of the clock mechanism itself," Hoeijmakers said in a telephone interview. Circadian rhythms influence functions ranging from sleep, and hormonal responses to jet lag and seasonal affective disorder (SAD), better known as the winter blues. A better understanding of how circadian rhythms work could allow scientists in the future to adjust the body's internal clock and to treat people who have disturbed biological rhythms because of a defect in the Cry proteins. Located in a region of the brain called the suprachiasmatic nucleus (SCN) the body's internal clock is governed from input from the retina of the eye where the proteins are found. But every cell also has its own clock, also controlled by Cry proteins."Every cell in every organ has an internal clock running as well," said Hoeijmakers. "We do not know exactly how that fits into the SCN driving the clock and probably steering a number of physiological processes in the body and the independent clock in the cell." In a research letter to the science journal Nature, the researchers described how they bred strains of mice lacking in one or both proteins. When the mice were tested in darkness the body clock of those lacking Cry1 ran faster while those without Cry2 slowed down. Mice without either had no natural rhythm to daily routines.