Investigations In Biology II Week 9: Fruits and Flying Seeds & Diversity of Leaves

Fruit ▪ Fruit: a ripened ovary of a flower usually encasing seeds ▪ However, self-incompatible genes render some species unable to set seed, but they produce fruits anyway (seedless fruits). Nurseries thus propagate fruit trees asexually, usually by grafting ▪ Fruit only occurs in angiosperms ▪ Includes what we refer to as ‘vegetables’ (eggplant, squash, peppers) ▪ Includes ‘nuts’ such as peanuts, pecans and cashews

Fruiting ▪ After fertilization the ovary develops into fruit ▪ Most other floral structures fall off or are reduced (but often some remain such as sepals on apples) ▪ Fruit tissue is mainly the pericarp (ovary wall). It has three layers: ▪ Exocarp: the peel ▪ Mesocarp: flesh of the fruit ▪ Endocarp: innermost layer surrounding the seed

Fruit tissue

Classifying fruits ▪ Simple: derived from a single ovary (e.g., peaches, apples, tomatoes) ▪ Fleshy (see above fruits) or dry (e.g., acorn). There are 2 types of dry fruits: ▪ Dehiscent fruits split apart at maturity (e.g., bean pods, hibiscus, iris) ▪ Indehiscent fruits do not split at maturity (e.g., sunflower seeds, corn) ▪ Compound: derived from many individual ovaries. There are two types, based on the number of flowers involved in producing the fruit ▪ Aggregate (one flower) include blackberries, strawberries, raspberries ▪ Multiple (many flowers or an inflorescence) include pineapples, mulberries

Classifying fruits

Seed dispersal ▪ Fruits not only protect the seeds, they can assist in dispersal upon maturity ▪ Fruits may depend on wind, water, or animals for seed dispersal ▪ Some seeds requiring animal dispersal produce sweet, nutrient rich flesh (e.g., blackberries, persimmons) ▪ Others hitch-hike instead of encouraging consumption (e.g., beggar lice) ▪ Others are designed to be wind dispersed (e.g., cat-tail, red maple) ▪ Others float for dispersal (e.g., coconuts) ▪ Still others explode when touched (e.g., touch-me-nots)

Seed dispersal

Touch me not seed pods exploding

Diversity of leaves ▪ Vegetative structures of angiosperms include roots, stems and leaves ▪ Vegetative structures grow from the plant body at localized areas of cell divisions called meristems ▪ Apical meristems produce primary growth in length at the tips of roots and stems ▪ Lateral meristems produce secondary growth in girth (width)

Diversity of leaves ▪ Primary function of leaves is to produce food for the plant via photosynthesis ▪ However, in some plants photosynthesis can occur in stems or even roots ▪ Some leaves are also specialized for storage (bulbs), protection (spines) and even capturing insects in carnivorous plants (e.g., pitcher plants)

Leaf anatomy ▪ Outer surface of leaf are covered by an upper and lower epidermis ▪ Epidermal cells are transparent and secrete a waxy layer called a cuticle that retards water loss ▪ Guard cells are highly specialized epidermal cells that occur in pairs around stomata (stoma = singular) ▪ Guard cells open when they are filled with water which causes them to bow apart ▪ Stomata are used for gas exchange with the environment, essential for photosynthesis

Leaf anatomy – guard cells

Transpiration ▪ When stomata are open, CO2 enters and O2 exits the plant ▪ Most of the water brought up into the plant through the roots is lost through the stomata = transpiration ▪ Although water is lost, this process is necessary to get water to the leaves form the roots ▪ Stomata are usually open during the day because gas exchange needed for photosynthesis, and closed at night to conserve water when photosynthesis is not occurring ▪ Most stomata are on the underside of horizontal leaves (they are on both sides of vertical leaves such as grass)

Mesophyll ▪ The epidermis surrounds the mesophyll ▪ The mesophyll is usually divided into 2 layers: ▪ The column-shaped cells of the palisade parenchyma make up the upper layer. This is the main site of photosynthesis ▪ The loose irregular cells of the spongy parenchyma make up the lower layer. It facilitates gas exchange between the leaf and the environment

Mesophyll ▪ Water & nutrients transported from the stems arrive via the veins or vascular bundles which are surrounded by bundle sheath cells ▪ Veins are composed of xylem and phloem ▪ Xylem is in the upper part of the vein and delivers water and nutrients to the leaf from the stem. ▪ Phloem is located in the lower part of the vein and transports organic products (usually sucrose) from the leaves to other parts of the plant

Mesophyll

Leaf arrangement & morphology ▪ Most leaves are made up of a petiole and a blade ▪ The petiole attaches the leaf to the stem. The blade is the broad flattened portion of the leaf ▪ Some leaves do not have a petiole (e.g., goldenrod) – these are said to be sessile ▪ Leaves may have a single blade (simple leaves) or be made up of several blades (compound leaf). Compound leaves have leaflets ▪ Axillary buds are found where the petiole emerges from the stem. These buds do not form at the base of a leaflet, and so can be used to distinguish between simple and compound leaves

Leaf anatomy

Leaf arrangement ▪ Pinnately compound leaves bear leaflets on either side of an extension called a rachis ▪ Palmately compound leaves bear leaflets that are all attached at a common point at the tip of the petiole

Leaf arrangement ▪ Leaves are produced by apical meristems at regular intervals along stems called nodes ▪ With alternate arrangement there is only one leaf per node on a stem ▪ With opposite arrangement there are 2 leaves at each node located at opposite sides of the stem ▪ With whorled arrangement there are 3 or more leaves per node

Leaf morphology ▪ Venation refers to the arrangement of veins in the leaf, and acts as a skeleton that reinforces the shape of the leaves ▪ Parallel venation exhibits the major veins running parallel to one another in a leaf (e.g., grasses, iris, cattails, corn) ▪ Net venation shows the major veins branching and smaller veins forming an interconnected network (e.g., roses, oaks, sunflowers) ▪ Palmate netting: several major veins radiate from one point (e.g., red maple) ▪ Pinnate netting: veins branch off along one main vein (e.g., willow)

Today ▪ Fruits Exercise 1. Observe the fruits provided. Use diagram 7-2 to determine each fruit type. Record your observations in Table 7-1 ▪ Leaves Exercise 3. Key out and match the dry and fresh plant specimens from SE Louisiana to the individual index cards. Use drawings or your phone to get pictures for studying later ▪ Fruits Exercise 2. Designing a wind-dispersed seed competition. You will work in groups of 2-4 to design a wind-dispersed fruit using a sunflower seed and various materials. The competition will be held from the balcony on level 3