Download Lab 10-Adaptations

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.
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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 _____________________________________________
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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 http://botany.cz/en/trichomes.
Image D from July 11, 2010 Naturalist Newsletter published on www.backyardnature.net.
Sketch and label three types of trichomes from your direct observations below.
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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.
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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? ____________________________
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