LAB 10: PLANT ADAPTATIONS

Transcription

1 LAB 10: PLANT ADAPTATIONS In this lab, you will 1. Identify the origin of modified structures and the environmental pressures that have led to the evolution of these adaptations (in the greenhouse with your TA). 2. Compare and contrast different types of trichomes. 3. Compare and contrast leaf cross-sections of plants adapted to dry vs. aquatic conditions. 1-SHOOT MODIFICATIONS The following paragraphs include the most common definitions. Rhizome. The term rhizome is most commonly applied to horizontally growing underground shoots. Rhizomes may be fleshy or relatively thin. They usually have reduced scale-like leaves, which are associated with axillary buds. Tuber. A tuber is the swollen tip of a horizontally growing underground rhizome. It contains abundant storage parenchyma (packed with starch or other substances). Potatoes and Jerusalem artichokes are typical tubers. The "eyes" of such tubers are the nodes where inconspicuous scale leaves and axillary buds are located. Corm is a modified shoot with swollen internodes consisting mostly of storage parenchyma and often with papery scale leaves. Usually the term corm refers to underground, modified shoots that develop in a vertical position at the base of an aerial shoot (contrasting with the tuber that develops at the tip of a horizontally-growing rhizome). At the beginning of the growing season a new shoot, often with a terminal inflorescence and one or more leaves, grows from the corm. The parental corm usually shrinks in size while, simultaneously, a new corm develops from an axillary bud on the old corm. As in the case of tubers, the axillary buds of a corm may develop into smaller versions of the parental corm. They are easily detached and used in propagation. Bulbs are bud-like structures in which food is stored in fleshy leaves (instead of in internodal stem tissue as in tubers and corms). Bulbs are not common in the dicotyledonous flowering plants, but are very common in the monocots. Onions are typical bulbs. When a bulb resumes growth, producing an elongate shoot, one or more axillary buds of the bulb will thicken into new bulbs. Cladodes are shoots that appear flattened and leaf like. Only very careful inspection reveals their true nature. You can look at Ruscus in the greenhouse to see this type of shoot. Caudiciforms are usually defined as those succulents in which there is a perennial storage organ, the "caudex", that is distinct from the weak and thin photosynthetic branches, often climbing or prostrate, that are present during the growing season. They abscise during the dry season, leaving only the caudex. 104

2 Stem & Leaf Modifications Examine the specialized stem, leaf and root modifications in the greenhouse. In each case, what organ has been modified, and what are the functions of each of these modifications? Fleshy stems of cacti and succulents Fleshy leaves of succulents Spines of cacti Parsnip root Potato tuber Stolons (runners) Rhizomes Bulbs Thorns Tendrils Prickles on blackberry Pitchers of the pitcher plant Tanks Bracts Cladodes Other 105

3 2-Epidermal adaptations Plants have a wide variety of specialized trichomes. Examine the leaf surfaces of the plants provided with the dissecting microscope. Trichomes on leaves and stems may reduce water loss and/or reduce herbivory. The unbranched trichomes of geranium are straight and those of bean are hooked. Compare the velvety feel of geranium leaf with the sticky feel of bean leaf. Some plants, such as Eleagnus (Russian olive), have elaborate branched trichomes. Trichomes may also secrete a variety of compounds. Examples of plants with secretory trichomes include tobacco, Drosera (sundew), Dionea (Venus fly trap), and Limonium (sea lavender). A B C D Examples of trichomes: A) Secretory trichome of Pelargonium (Geranium), B) Trichome of Urtica dioica (stinging nettle), C) branched trichome in Verbascum, D) hooked trichomes in Gronovia. Images A,B,C from Josef Špaček, Invisible Life 11: Seed Plants, at Image D from July 11, 2010 Naturalist Newsletter published on Sketch and label three types of trichomes from your direct observations below. 106

4 3-Anatomical Adaptations in the Leaf Leaves are internally modified in many ways depending on the type of environment in which the plant is found. You will compare in more detail two leaf anatomies: those from plants adapted to grow in moist (mesic) environments (Syringa) and leaves from plants adapted to dry (xeric) environments (Nerium). You will also be introduced to leaf anatomy from an aquatic plant (Nymphaea). Mesic leaf: Obtain a slide of a leaf cross section of Syringa. Illustrate below and label: upper and lower epidermis, stomata, mesophyll (spongy and palisade), vascular bundles with xylem and phloem. Xeromorphic leaf Obtain a slide of a leaf cross section of Nerium (Eudicot). This leaf cross section will be similar in appearance to Syringa, but there are some interesting differences. First, you should recall that Nerium is a plant that likes warm, dry conditions (this is why they are sometimes used as shrubs along highways in southern California). The leaves are toxic. Nerium oleander contains the toxins oleandrin and nerioside, very similar to the toxins in foxglove (Digitalis). Note the special features of this leaf: mesophyll cells containing calcium oxalate crystals, multilayered epidermis (upper and lower), and stomata in stomatal crypts with trichomes (adaptation to dry, hot climate). Locate guard cells. Cross-section of Nerium oleander leaf. 107

5 Aquatic (hydric) leaf: Obtain a slide of a leaf cross section of Nymphaea (water lily, basal Angiosperm). - This plant lives in an aquatic environment. What special feature does it have for growth in an aquatic environment Find branched or star-shaped brown, thick-walled sclereids in the leaf tissue. What might their function be in terms of leaf structure? 108