Endterm Botany: Leaves and Photosynthesis PDF

c. Leaf base - the part near the petiole. In monocots, the leaf base is expanded into a CHAPTER 9. LEAVES structure called the leaf sheath, which End Term partially or compl...

c. Leaf base - the part near the petiole. In monocots, the leaf base is expanded into a CHAPTER 9. LEAVES structure called the leaf sheath, which End Term partially or completely covers the stem. In many dicots, the leaf base bears two lateral outgrowths known as stipules. Plants may be distinguished from one d. Leaf blade/Lamina - the green, flattened another through leaf morphology. The portion, which performs most of the variety of shapes, sizes, colors, and textures photosynthesis in the plant. A strong or of leaves is almost infinite. You will be thick vein, known as the midrib, runs amazed at how varied plant leaves are. through the leaf blade from its base to the apex. The midrib branches into thinner The leaf may be regarded as the flattened, lateral veins that give rise to still thinner lateral outgrowth of the stem or node with lateral veins called veinlets. a bud in its axil. This is the primary organ for photosynthesis in plants. All leaves originate in the shoot's apical meristem as a bulge of tissue called leaf primordia. Leaf primordia develop into leaves through cell division, growth, and differentiation. EXTERNAL MORPHOLOGY 1. A typical leaf consists of the following parts, each with its own function: a. Stipules - outgrowths, usually paired at 2. Phyllotaxy - leaf arrangement on the stem the base of some dicot leaves. Leaves with Leaves are attached to the stem in different stipules are stipulate, and those without arrangements: stipules are exstipulate. a. Alternate or Spiral - a single leaf is b. Petiole - the stalk of the leaf that is present at each node. This is the most attached to the stem. When the petiole is common arrangement (e.g., lanzones, absent, a leaf is called sessile; and when it is sunflower, five fingers, santol, mango). present, it is called petiolate. b. Opposite - two leaves arise from each node, opposite each other. They may be at right angles to the upper or lower pair (e.g., a. Unipinnate - leaflets are attached guava, japanese bamboo, and makopa directly to the rachis (e.g., Cassia, [Syzygium sp.]). kamias, Pteris [fern]). c. Whorled - there are more than two b. Bipinnate - when the compound leaf leaves at each node in a circle or whorl (e.g., is twice-pinnate. The primary rachis yellow bell [Allamanda nerifolia], dita branches into what is known as the [Alstonia scholaris]). secondary rachis, tO which the leaflets are attached (e.g., 3. Leaf Types caesalpinia, makahiya, acacia) a. Simple leaf - consists of a single blade, c. Tripinnate - there are rachises of the which may be entire, lobed, or cleft but not first, second, and third orders. down to the midrib (e.g., santol, banana, Leaflets are attached to the tertiary gabi). rachis (e.g., malunggay). b. Compound leaf - has a blade divided into a number of segments called leaflets in various ways (e.g., mahogany, five fingers, rain tree). b. Palmately compound leaf - the leaflets are attached to the tip of the petiole from which they radiate (e.g., octopus tree An axillary bud is present in the axil of a [Brassaia actinophylla), five fingers simple and a compound leaf, but it is never [Schefflera elliptical. present in the axil of the leaflet of a compound leaf. 4. Leaf venation - refers to the arrangement of veins and veinlets in the leaf. There are two types of compound leaves: There are two types: a. Pinnately compound - the midrib, known as the rachis, bears leaflets arranged in a a.Netted or reticulate - in this type, there linear sequence. They may be arranged in may be one or more large veins from which an alternate or opposite manner and may smaller veins branch out and interconnect, be of the following types: forming a network pattern. This type of venation is a characteristic of most dicots. 1. Upper epidermis - a single layer of similar This type of venation may be: cells covering the upper surfaces of the leaf, Pinnately reticulate - has one generally with a waxy cuticle, to prevent principal vein or midrib present, excessive loss of water from the leaf from which smaller veins and their surface. It consists of closely packed cells branches spread out in all directions and usually contains no chloroplasts. in the blade. Stomata are usually absent. Palmately reticulate - has several 2. Mesophyll - the ground tissue lying large veins radiating from around between the upper and lower epidermises. the tip of the petiole. It consists of two types of tissue: a. Palisade parenchyma - composed of b. Parallel venation - veins run parallel to long, cylindrical, compactly arranged cells each other. This type of venation is a with numerous chloroplasts. Palisade cells characteristic of monocots. may be one to several layers thick. Their main functions are to absorb light and manufacture food. b. Spongy parenchyma (spongy mesophyll) - the lower part of the mesophyll, consisting of oval, circular, or irregularly shaped cells that contain chloroplasts. There are intercellular spaces present in this layer. INTERNAL ANATOMY 3. Veins (vascular bundles) - cylindrical strands of vascular tissues that occur mainly The leaf's internal structure is closely in the mesophyll. Veins of various sizes related to its many functions. Most leaves (vascular bundles) are in all directions for have an outer layer of epidermis, which the distribution of water and food. Each protects the internal chlorophyll-containing vascular bundle consists of xylem and cells involved in photosynthesis and the phloem. Xylem cells have angular walls and vascular systems that bring in water and are always present toward the upper minerals and transport photosynthetic epidermis. Phloem cells are less distinct, by-products to all parts of the plant. thin-walled, and located below the xylem, toward the lower epidermis. Internal Morphology of a Dicot Leaf 4. Lower epidermis - similar to the upper epidermis, except that it has a thinner cuticle and its pores, or stomata, are larger A cross-section of a dicot leaf blade shows in number. These small pores control the the following parts: exchange of gasses in and out of the leaf and the loss of water vapor. Each stoma is THE STOMATA surrounded by two kidney-shaped cells Plants must be able to prevent excessive called guard cells. water loss to prevent wilting. Ninety percent (90%) of water loss in plants occurs through the stomata. The closing and opening of the stomata are, therefore, very Internal Anatomy of a Monocot Leaf important for the plant. Stomata are mostly located in the lower epidermis of the leaf, where temperature is lower, and where Next to the epidermis are the collenchyma clogging of the stomata is avoided. Up to cells, followed by parenchymatous tissues. 10,000 stomata can be found for every square-m of the lower epidermis. The internal structure of a monocot leaf is Guard cells regulate the size of the stomata. similar to that of a dicot leaf in having an When suthcient water is available, the upper and a lower epidermis, mesophyll, guard cells take up a lot of water and and vascular tissue, among others. become curved, as does an inflated balloon, However, they differ in some aspects. The thus opening the stomata. upper and lower epidermis may have an equal number of stomata, and the cuticle The stomata are the structures through may be of the same thickness. The which gases enter and move out of the leaf. mesophyll of monocots is not generally Several factors are responsible for the differentiated into palisade and spongy opening and closing of the stomata: mesophyll. It mostly consists of spongy parenchyma cells with chloroplasts that are 1. response to internal concentrations irregularly shaped. of CO 2. high temperatures Cross-section of the Midrib 3. blue light of the visible spectrum The midrib is the central or main vein of a 4. low humidity leaf. It is bound by the upper and lower 5. hormones such as abscisic acid epidermis. Next to the epidermis are the (ABA). collenchyma cells, followed by the parenchyma tissues. When the guard cells have high ABA levels, they lose water, and the pore closes. Water The vascular bundles are crescent-shaped or uptake in plants is brought about by the circular in cross-section. In the circular influx of K* ions into the guard cells, vascular tissues, the phloem surrounds the followed by water molecules, resulting in xylem. the opening of the stomata. The exit of K* ions also lead to the exit of water, making the guard cells shrink and resulting in the closure of the stoma. When guard cells are turgid, the stoma opens; when flaccid, The stoma closes. Stomata are usually open during the day and closed at night. However, this may not always be the case. Some plants adapt to existing environmental conditions by closing their stomata on hot summer days (photorespiration), such as C4 and CAM plants. in the early morning when grassy fields are moist with dew.  PHOTOSYNTHESIS The Nature of Light and Pigments Photosynthesis (Photo meaning light; synthesis meaning building up) is the Light is the energy carried by particles biological process by which green plants called photons. Photons have wavelengths, convert light energy into chemical energy to and the longer the wavelength is, the less be stored in food molecules. For energy is stored. When light strikes an photosynthesis to occur light, chlorophyll object, it may be absorbed, reflected, or pigments, water, and carbon dioxide must transmitted. be present. Reflected light gives an object its color. This is the reason why we see leaves as green. Except for a few exceptions, all living things Green light rays are not absorbed by the are dependent on the products of pigments and instead are reflected back to photosynthesis for their energy, the raw our eyes. materials necessary for growth and repair, Absorbed light can provide the energy that and the oxygen necessary for life. This drives the chemical reactions, just like in shows the interdependence between plants photosynthesis. and animals. Light energy absorbed by chlorophyll generates electrons and protons from water molecules. Some of this light energy is stored in a compound called adenosine triphosphate (ATP), and oxygen is released as a There are several pigments present in plants by-product. The ATP drives the reduction that help in photosynthesis. forward with combined carbon dioxide from e and H+ to form water and carbohydrates Chlorophyll absorbs violet, blue, and red or other compounds at the same oxidation light, but reflects green. level. A simple equation of the process is The two most common types of chlorophyll given below: are chlorophyll a and chlorophyll b, which are both indispensable in the light reaction. Carotenoids absorb blue and green light and are visible in the fall. When plants stop producing chlorophyll, leaves take on a yellow or red color. Phycocyanins absorb green and yellow light. Xanthophyll, on the other hand, is a type of carotenoid that very unstable and reactive. This light energy reflects yellow light in leaves in the fall. is converted to ATP and another energy As a process, photosynthesis is divided into carrier molecule called NADPH. Each two phases: photosystem has only one chlorophyll light-dependent molecule and hundreds of chlorophyll b light-independent reactions. molecules and carotenoids that help harness light energy. Light-Independent Reactions The products of the light-dependent Chloroplast reactions, ATp and NADPH, are necessary for the light-independent phase to occur. This phase is also known as the Calvin Cycle. Light-Dependent Reactions In this stage, ATP and NADPH are stored in a more permanent form as sugar in a series of Light-dependent reactions take place on chemical reactions involving carbon fixation. the surface of the thylakoid membrane of Carbon dioxide is now reduced to glucose. the chloroplast. Chlorophyll converts These reactions are light-independent in sunlight energy into chemical energy (ATP that they can occur in the absence of light, and NADPH). This phase cannot take place as long as ATP and NADPH are provided. The in the absence of light, in contrast to the diagram below will help you understand the light-independent reactions. process better. The chemical reactions of photosynthesis take place within the chloroplast. There are two reaction centers (antennal centers) in the chloroplast: photosystems I photosystem II. These are large patches of chlorophyll molecules arranged together with their heads parallel. When a photon of light is absorbed by the chlorophyll, one of its electrons is boosted to a higher energy level, where it becomes MISS PPT The mesophyll Inside the leaf that is What Is a leaf rich in chloroplasts Leaf is an organ of a vascular plant, and the The arrangement of veins (vascular food making factories of green plants. tissue) Leaves come in many different shapes and sizes. Types of leaf 1. Simple leaf: has one single blade Leaf 2. Compound leaf: has more 2 External Features Primary and secondary Leaf blade - Flattened part of the leaf Types of compound leaf Petiole - Leaf stalk palmately compound - leaflets radiate Stipules - Leaflike appendages at from the end of the stem like a the base of the leaf. pinnately compound - leaflets, are arranged on either side of a central man Leaves can be simple. They are made of a stem single leaf blade connected by a petiole to the stem. An oak leaf is an example. According to veins Parallel-veined leaves: the veins run A compound leaf is a leaf made up of at the same distance to others like in separate leaflets attached by a petiole to canes. the stem like a palm leaf. Pinnate leaves: there is a main vein called midrib, from which the other Photosynthesis - The process of nerves derive, remembering a photosynthesis takes place in the leaf of the plume. plant to produce glucose. Palmate leaves the nerves derive from the main point such as the fingers do in the palm of the hand. Leaf anatomy A leaf is a plant organ and is made up of a Palmately compound leaf collection of tissues in a regular 1. Unifolite (citrus) organization. 2. Bifolite (Gulmohar leaf) 3. Trifolate (Trifolium) The major tissue systems present are: 4. Quarrifolite (Marsilea Quadrifolia) CallThe epidermis that covers the 5. Multifolate (Cottan tree) upper and lower surfaces