Outline • Storage organs • Potato • sweet potato • Yam • cassava • Chap 14
Underground Food Storage • One successful plant strategy is to keep important parts of the plant below the ground, safe from most animals and weather, and send up disposable stems for photosynthesis. • Food reserves are needed to build the aerial parts of the plant before photosynthesis can take over. • Seeds have food stored in the endosperm or cotyledons. • Plants that reproduce vegetatively often store starch in modified roots or stems below the ground. • A common strategy: plants as biennials. This means they have a 2 year life cycle. Carrots, for example. • First year: concentrate on vegetative growth, • Winter: store the food they made underground • Second year: send up a flower stalk to reproduce.
Types of Storage Organ • Modified stems: • Stolons (also called runners) are aboveground horizontal stems that produce new plantlets (stems and roots) are their nodes. Examples: strawberry and crabgrass. • Rhizomes are horizontal stems that are underground, often just below the surface. Examples: ginger and iris • Tubers are swollen tips of rhizome buds. Examples: potatoes
More Storage Organs • More modified stems: • Bulbs are underground stems surrounded by fleshy leaves that hold the starch, protected by a layer of papery leaves. Examples: onions, tulips, daffodils. • Corms are underground stems that store food in the stem itself, not in modified leaves (as bulbs do). Corms are surrounded by protective papery leaves. Examples: crocus, gladiolus, taro.
Modified Roots • Tuberous roots are modified fibrous roots that hold food. Examples: tuberous begonias, sweet potato. • Taproots can be enlarged as food storage organs. This often happens in biennial plants, which store one years’ food production belowground, then use it to grow reproductive structures the next year. Examples: carrot, turnip.
Potato • Solanum tuberosum, the white potato, is a member of the nightshade family (Solanaceae) along with tomato, chile pepper, and tobacco. • The potato is a New World crop, native to the Andean highlands of Peru and Bolivia • Potato cultivation was widespread in North and South America before Columbus, with lots of varieties. • Today, almost all potatoes are descended from a subspecies native to Chile. • Potato is a tetraploid, the product of a cross between two diploid (and much less productive) species. • Potatoes are eaten worldwide, and they are the fourth biggest food source, after rice, wheat, and corn.
Potato History • The potato was the main food energy source for the Inca Empire in Peru before Columbus. • After harvesting, potatoes were exposed to the cold night air to freeze, then mashed and dried (a low-tech form of freeze-drying). This kept them edible for many months. • The Spanish introduced the potato to Europe, but it was not immediately popular. • Partly because it is in the same family (and looks a lot like) deadly nightshade! • Most parts of the potato plant, other than the tuber, are poisonous • Potato cultivation slowly spread across Europe. It had two advantages: • It grew well in cold conditions. The European climate was undergoing the Little Ice Age between about 1600 and 1850, when average temperatures were lower than before or after, resulting in many crop failures • The tubers could be hidden underground, out of sight from tax collectors and marauding soldiers.
Irish Potato Famine • In the 1800’s, Ireland was ruled by England, with laws that prevented Catholics (most of the Irish) from getting an education or owning land. Most Irish people rented tiny plots of land from landlords who lived in England. The standard of living was very low, and tenants could be evicted at any time. • Rent was paid by cultivating the best land with wheat and oats, which were shipped to England • In contrast, the English people were rapidly becoming more prosperous due to industrialization. • Potatoes produced sufficient calories that a family could live on 1 acre of land, with a milk cow or a pig. No other crop could produce enough food in such a small space. • Potatoes are propagated vegetatively, by planting the “eyes” of the tubers. This means that all potato plants in a field are genetically identical. • The potato blight, a fungal-like protist called Phytophthora infestans , appeared in 1845. It came from Mexico. There was very little genetic variation among Irish potatoes, so nearly all got infected. It was called “late blight” because it destroyed the crop just before it could be harvested.
More Potato Famine • The blight continued until about 1850 • Half the potato crop lost in 1845 • 3/4 of the crop lost in 1846 • Political ideology in England allowed little aid to be sent to the starving Irish. Also, they weren’t paying their rent, so many were evicted. • Many Irish died of starvation and disease. The population of Ireland in 1911 was half that of 1850. • Many emigrated to the US and elsewhere. About 1/3 of all Americans today have some Irish ancestors. • The blight itself was stopped by the use of new resistant strains of potato, and by the development of an effective fungicide.
Potato Cultivation • Most potatoes are grown from the “eyes” (buds) of the tubers. This makes the offspring clones, genetically identical to the parents. • The lack of genetic variability in potatoes makes them susceptible to widespread diseases. • In contrast, most other plants are grown from seed, which results from sexual reproduction. The genetic variability introduced by randomly combining traits from two different parents means that there are usually some plants resistant to any disease. • “Seed potatoes” are cut pieces of the tubers, which will grow into whole plants. They are tested and certified as disease free: many diseases are passed through the tubers • Potato breeders grow the seed and hybridize them to produce new varieties. • They like a cool climate, and tuber formation is inhibited at temperatures above 85oF. In the US, most potatoes are grown in Idaho, eastern Washington and eastern Oregon.
Uses of Potatoes • Most potatoes are eaten by people: • Fresh potatoes are baked or boiled • Cut up into french fries • Sliced thin and baked to make potato chips • Some use as animal feed • Potato starch is used in the food industry to thicken soups and sauces, and in the manufacture of paper.
Sweet Potato • Despite the name, the sweet potato (Ipomoea batatas) is not closely related to the familiar white potato. Sweet potatoes are in the morning glory (Convolvulaceae) family. • Yams are another, entirely different plant, native to Africa. Most of what is marketed as “yams” in the US are really sweet potatoes that have a darker orange color than other sweet potatoes. • Sweet potatoes are native to the New World, probably domesticated in the region between the Yucatan peninsula and Venezuela. • However, sweet potatoes are also found in Polynesia. • Sweet potatoes grow in tropical and warm temperate conditions. It needs significant rain, but can’t stand waterlogging: the tubers rot. • Mostly grown in southern states of the US: North Carolina and Louisiana are the main producers • Also heavy users: Papau New Guinea and New Zealand (Pacific Ocean), and Rwanda, Burundi, Uganda (in Africa) • Lots grown in China, mostly as animal feed
Spread of the Sweet Potato • One big mystery about world history is why the sweet potato plant was cultivated in both South America and Polynesia before the European invasion. Also, the two groups use roughly the same word for it: the Peruvian word is “kumar” and the Polynesian word is “kumara”. • Polynesia was settled from western Asia, probably the Malay peninsula, starting about 2000 years ago. • Easter Island, the easternmost pacific island, was settled about 1500 years ago, roughly the same time as Hawaii and New Zealand. • These islands were probably the last places settled by humans on Earth. • All done using double-hulled canoes, with very fine navigation skills., transmitted orally.
Possible Polynesian-South American Contact • In addition to the common use and words for the sweet potato, chickens were also (apparently) found in both Polynesia and South America before Columbus. Also, there are other common words. • Genetic and archeological evidence is still equivocal. • It’s a long way from Easter island to South America, but it is not an impossible voyage for the Polynesians. • Thor Heyerdahl, a Norwegian, thought that maybe South Americans the Pacific to Polynesia on balsa wood rafts. He built one, the Kon-Tiki, and sailed it to the island of Raroia in 1947, which took 101 days. He then wrote a book, which became a best seller. • But, his theory that Polynesia was settled from South America isn’t viable in light of modern genetics and archeology. • And, it’s also not proof that there ever was any contact • It has been suggested that the stone gods on Easter Island had seeds in their mouths when the space aliens brought them to Earth. The gods then spat the seeds eastward to South America, and westward to Polynesia. • Most authorities think this theory is unlikely.
Yam • There are several species of cultivated yam, in the genus Dioscorea, which is part of the larger yam family (Dioscoreaceae). • They are found in humid tropical regions throughout the world, and have domesticated several times independently. In parts of Africa they are a major staple crop. • The plants are perennial vines, up to 40 feet long. • Propagated asexually, using pieces of tubers with buds. • The yams themselves are tubers, which range in size from potato to up to 40 kg (88 lb) and 10 feet long. They can be deeply buried, so harvest is a challenge. • Mostly starch, with some protein and vitamins.
Yams and Hormones • A number of important human hormones are steroids: they have a characteristic 4 ring structure. • Male and female sex hormones, stress hormones, ion balance hormones • Yams produce a chemical compound that resembles steroid hormones. • Probably used to defend against animals: used as a fish poison in some places. • Taken as a contraceptive by natives in Brazil. • In the early 1950’s, the steroid hormone cortisone was first isolated, from the adrenal glands of 2,200,000 pigs. Obviously not cost-efective. • Starting with the compound from yams, cortisone could be produced for $2 per gram, incredibly cheap. • Also other steroids including female sex hormones that went into birth control pills, and male sex hormones used (illegally) for body building. • Nowadays, synthesis is done from simpler materials, eliminating the need for yams.
Yams and Twins • Among the Yoruba of Nigeria, the rate of twins is 5-10 times higher than is common in the US. They also eat lots of yams there. Maybe that’s why they produce so many twins. • However, this is just a correlation: twins and yams occur together in one part of the world. Is it a cause-and-effect relationship? • Maybe the twins are caused by some genetic factor among the Yoruba. • Maybe it’s something else they eat, or some other environmental factor • Maybe the high rate of twins isn’t even true • Google shows lots of websites, but none of them has any references to scientific literature. • Scientific papers are peer-reviewed. This doesn’t guarantee their accuracy, but at least someone with a skeptical eye had to agree that the conclusions were supported by the data. • My opinion: I think it is unlikely to be true. If it were true, other yam-eating cultures would also have more twins, and probably some birth control pills would affect twinning.
Cassava • Also called manioc. Manihot esculenta, a member of the spurge (Euphorbiaceae) family. • Not common in the US, except as tapioca pudding, but the fourth most important calorie source in the world (after rice, wheat, and corn), primarily in the tropics. • A New World crop, originating in Brazil. Spread by the Portuguese to Africa and India, then later to Indonesia and the rest of Asia. • The plant is a perennial shrub, with tuberous roots. • Propagation through stem cuttings, not the tuberous roots. • Also, it grows quite well from seed, meaning there will be a lot of genetic variation. • Grows well with seasonal rainfall: it can survive 6 months of drought. • High in starch, low in protein: needs to be supplemented to avoid protein deficiency disease.
Cassava Processing • Cassava roots contain poisonous compounds called cyanogenic glycosides. When acted on by digestive enzymes, they produce hydrogen cyanide, which is very poisonous. • In some varieties, the poison has been bred out • For most varieties, the roots need to be processed by drying, soaking, boiling, grating, or fermenting (depending on the culture) before they can be safely eaten. • South American method: grate the roots, soak them in water, then strain them through a cloth. This washes out most of the poison. Then, dry the mash by roasting it. • Once processed, cassava can be stored for later use