Dropper bottle of distilled water
Dropper bottle of iodine solution (IjKI
Plants to examine tor leucoplasts and starch grains
(e.g., white potato, crown of thorns, banana-
Starch of arrowroot, potato, corn, ctc.
EXERCISE C: Storage Organs
At the start of the growing season, stored starch serves as the energy source for new aboveground growth. In stressed environments when conditions arc not favorable for photosynthesis, starch provides the plant with a ready supply of organic solutes. Any part of a plant may be adapted for storage. Some storage organs arc roots, while others are modified underground stems and leaves (fig. 13.3). (Note: For assistance in identifying the plant parts in the following exercise, refer to Laboratory Topic 4.)
Bulb of daffodil Compound microscope Corm of crocus Covcrslips Dissccting needles
Dropper bottle of iodine solution KI) Dropper bottle of distilled water Glass slides
FIGURE 13.3 STORAGE STEMS AND ROOTS.
The bulb is a modified stem with storage leaves. These white storage leaves make up die bulk of the bulb. The stem at the base of the bulb gives rise not only to leaves but also to roots. Pick off one white leaf and note its diin, translucent covering (epidermis). Take a small sample from the fleshy middle or mcs-ophvll of a leal" and place the sample on a glass slide. Add a drop of water and iodine solution. Cover with a coverslip. Using a compound microscope, view the slide under the low (10 x objective) power first, and then switch to high i40 x objective) power. Observe the numerous starch grains within each parenchyma ccll of the leaf.
If your bulb has sprouted, it is easy to recognize a third type of leaf. These arc the green foliage leaves, die ones that appear aboveground when the bulb is planted. What would their main function be:
Razor blades, single-edged or knives
Rhizome of ginger
Storage organs from other plants
Taproot of carrot
Tuberous root of spider plant
Tuber of caladium
1. Obtain a daffodil bulb and observe the papery brown coverings on the outside of the bulb. These are modified leaves that act as a protective barrier and deterrent to discourage microbes and other soil organisms. Note the veins running through these leaves. What is the venation pattern? Is this a monocot or dicot plant?
Next obtain and examine a crocus corm. As with the bulb, papery leaves on the outside form a protective barrier. Again, it is easy to sec the venation pattern of these leaves. What is the pattern? Is the crocus a monocot or dicot plant?
Now slice the bulb longitudinally. What does this resemble:
Now slice the corm longitudinally. Note that the corm is a solid stem, unlike the stem and storage leaves in the bulb. Take a small piece of the corm flesh and place it on a glass slide. Add a drop of water and a drop of iodine solution. Cover with a coverslip, and look for starch grains first under low (10 x objective) power and then under high (40 x objective» power. Record in worksheet 13-1.
3. The rhizome of ginger is the next storage organ to examine, t'nlike the bulb and corm. which arc vcr-
rically oriented, a rhizome shows a distinctive horizontal orientation. The rhizome is an underground horizontal stem. You mav also find adventitious roots
arising from the rhizome.
You can verify the starch storing capacity of the rhizome by taking a small sample of it and preparing a slide to observe the starch grains. Record in worksheet 13-1
4. Examine the tuber of a caladium. Tubers are enlargements found at the ends of some rhizomes. Examine closclv the "eves" of the tuber. Botanicaliv
speaking, the eyes arc collections of buds.
Would a tuber be a root or a stem? How do you
Sample a piece of the inner storage region of the caladium tuber, and examine it under the microscope for starch grains. Record in worksheet 13-1.
5. Obtain a carrot and cut it horizontallv. Note the ccntral corc of tissue cncircled by a large outer region. Have you seen this arrangement before? In what vegetative organ? What is this corc region called? Would a carrot be a root or a stem? How do vou know?
Sample a piecc of the inner storage region of the taproot of the carrot and examine for starch grains under the microscope. Record in worksheet 13-1.
EXERCISE D: The Starchy Staples
The crops callcd starchy staples arc equal in food value to the cereals and legumes. But, unlike the cereals and legumes, the starchy staples arc not exclusive to a single family. This vegetable group is so named because of the high quantities of starch sequestered within the underground storage organs. Although most of these crops arc tropical in origin, several are adapted to grow under temperate conditions.
Materials Needed for Exercise D
Dropper bottle of distilled water
Dropper bottle of iodine solution ' KJ)
Dropper botde of Sudan III
White potato varieties: round white, russet, long white, round red
1. White potatoes (Sola ?j urn tuberosum) originated in the Andean highlands of South America. Although tropical, the growth at the cooler temperatures of higher altitudes allows its widespread cultivation in temperate climates. Obtain and examine a white potato. Note the "eyes," which as discussed in Fxcr-cisc C, arc collections of buds at nodes.
Cut the potato in half and note the ring of tissue just inside the skin. This ring of tissue can also be seen in a potato chip. What is it called? WTiar is the large ccntcr region called? What vegetative organ (root, stem, or leaf) of plants has this organization?
What conclusion can you draw about the iden titv of the storage organ of the potato? Record your conclusion by continuing to till oui worksheet 13-2 (begun in Exercise C).
The most familiar varieties of the white potato belong to just four groups: round white, russet, long white, and round red. The round white is a multipurpose variety that works for all preparations—boiling, baking, or converting into chips, fries or flakes. The russet type is the classic baking potato, and its oblong shape is also ideal for processing into French fries. Long whites and round reds are sold as new potatoes because they are harvested early in the growing season while the skins are still thin. New pota toes are used for roasting, steaming, or boiling. View each type on display in the lab.
2. The sweet potato (Ipomea batatas) is the tuberous root of a vine in the morning glory family, llie sweet potato is native to tropical South America, where Christopher Columbus encountered it on his first voyage to the New World. The Arawak people of the Caribbean callcd it batatas, which became -potato" in English; and the same name was bestowed upon Solatium tuberosum because it too was an underground crop from the New World. The sweet potato is also often confused with the true yam, another tropical underground crop. In the United States, sweet potatoes and yarns are simply varieties of Ipomea batatas. The sweet potato has a yellower, drier, and starchier flesh than the yam, whose flesh is sweeter, moistcr, and more orange in color.
Obtain and examine a sweet potato. Note the orange color of its flesh. What orange plant pigment would account for diis color? What vitamin would you
expect to find in abundance in the sweet potato ?
Cut the sweet potato horizontally. Note the inner circle. What inner ring of tissue can be found in roots?
Take a small piece of the sweet potato flesh and prepare a slide to examine its starch grains. Add sweet potato to the chart in worksheet 13 1 (begun in Exercise B), and draw a few of the starch grains.
3. Cassava, Maniboi csculenta, is known by many common names, including manioc, yucca, mandioca, and tapioca. A tropical member of the spurge family, cassava is actually a very large tuberous root from a small tree or bush. Sweet and bitter varieties are classified according to the concentration of cyanogenic glycosides. Cyanogenic glycosides release deadly hydrocyanic acid. Sweet varieties have low levels of these glycosides, while bitter varieties have much higher levels that must be removed dirough extensive preparations to make the cassava safe to cat. Traditional methods of detoxifying bitter varieties vary among cultures, but may include one or more of the following processes: drying, grating, boiling, fermenting, and soaking. Most people are familiar with tapioca, in which the moistened starch of cassava is gently heated to form gelatinized beads. The tapioca pearls are then cooked with milk, eggs, and sugar to make pudding.
Take a sample of cassava or its starch ro view the starch grains. Because the starch grains arc some of the smallest in the plant world, cassava starch and its products are easily digestible and a valuable food for infants and invalids. Record and draw cassava starch grains in worksheet 13-1.
EXERCISE E: What are You Eating?!
Although the estimated number of edible plants approaches 50,000 ¿pedes, less than 300 have been widely cultivated. Of these, wheat, rice, and corn con tribute nearly two-thirds of the plant derived calorics in the human diet. This means there arc literally thousands of potent ially useful plants that most of humanity has yet to discover. Government organizations and researchers are constant!) on the lookout for plants that arc little-known but potentially uscftil to bring to die greater attention of consumers in the world's marketplace.
A variety of little known plant foods (amarandi, arrowroot, chcrimoya, Jerusalem artichoke (sunchoke), jicama. malanga, quinoa, sunchokc and tomatillo)
Examine and sample each of the litdc-known food plants available in the lab. Using your knowledge of the anatomy of plants, decide what plant part is represented by die following plants, and record your observations in worksheet 13-3. If ncccssary, refer to Laboratory Topics 4 and 7 to ascertain what part of the plant is eaten. Draw starch grains in worksheet 13-1.
(henopodium quinoa) is cooked like ricc or ground into a flour that can be mixed with wheat flour to make bread and other baked goods.
Another noncereal grain is amaranth, one of several species within the genus Amaranthus. These nongrass grains can be toasted, boiled, popped, or ground into a flour that is mixed with wheat to make a variety of baked goods. Its protein content rivals that of the ccrcal grains.
Examine quinoa and amaranth. Botanically, what parts of these plants are actually eaten or ground for flour? Sample foods made from quinoa and amaranth.
6. Two fruits that have been making inroads in North American supermarkets arc the tomatillo and the chcrimoya. Tomatillo, or the husk tomato, is a staple of salsa verde, the green salsa served with Mexican dishes. It can also be sliced and eaten raw. The husk tomato
Pbysalis ixocarpa is so callcd because of its resemblance to a tiny tomato in a papcrlike husk. Sample the tomatillo and determine its fruit type. Chcrimoya, or custard apple, was known by the Incans. The fruit grows on a tree (Annona cherimola) that is native to the uplands of Peru and Ecuador. The fruits are chillcd, and then the custardlikc flesh is scoopcd out and eaten alone or with cream. Sample the delicious chcrimoya, and determine its fruit type.
TERMS TO KNOW
amylopectin 171 amyloplast 172 amvlosc 171 buib 174 condensation reaction 171 corm 174 Icucoplast 172
monomers 171 polymer 171 polymerization 172 rhizome 175 starch grains 172 taproot 175 tubers 175 tuberous roots 175
Epstein, H. 1996. ('rippling harvest. Natural History 105(7):12-15.
Lcvctin, E., and K. McMahon. 1999. Plants and society 2d ed. New York: McGraw-Hill Companies, Inc.
ON THE WEB
Cal Photos, Berkeley Digital Library Project http://dlp.CSierkeley.EDU/photos
LAB SECTION NUMBER
WORKSHEET 13-1 EXERCISES B. C. AND D: STARCH GRAINS. STORAGE ORGANS
AND STARCHY STAPLES
ÏST 1 3 ! Si
LAB SECTION NUMBER
WORKSHEET 13-2 EXERCISES C AND D: STORAGE ORGANS AND STARCHY STAPLES
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