seed development occurs directly after
fertilization (see p. 115). as discussed
in chapter 1, angiosperms (flowering plants)
produce enclosed seeds, protected within a
carpel, and gymnosperms produce “naked”
seeds with no special structure to enclose them.
naked seeds are typically (but not always)
carried uncovered on the bracts of the cones.
as the seeds of flowering plants mature, the carpel also begins to ripen into a hard or fleshy structure; as a whole, this unit becomes the fruit (see p. 78). The fruit may remain a combined entity right up until the moment that the seed(s) within it germinate, or a seed within it may separate from its housing. fruit that opens to release its seeds is termed dehiscent, whereas fruit that does not open is indehiscent. The exact mechanism depends on the life strategy of the plant concerned, and it is largely to do with protection and dispersal of the seed (see pp. 78–80).
to gardeners, the word “seed” can be extended. “seed” potatoes are, in fact, stem tubers, and some of the things that gardeners sow are actually dry fruits (which contain the seeds), such as grass “seed” or the corky fruits (sometimes called seed clusters) of beetroot (Beta vulgaris). Most seed for commercial production is carefully screened and “cleaned” from nonseed debris, so that when you buy a packet of seeds that is essentially what you get.
The purpose of seeds
In evolutionary terms, seeds are an important innovation that has led to the huge success of angiosperms and gymnosperms (collectively known as the spermatophytes—the seed-bearing plants).
one main advantage they have over the spores of lower plants (such as ferns and mosses) is that they are, as a rule, far more durable, being able to endure extended periods of dormancy and harsh environments.
although they are not the only means by which plants reproduce themselves, seeds are often able to disperse themselves over great distances (take, for example, the tufted airborne seeds of dandelions (Taraxacum) and enable a plant to colonize new, faraway locations. seeds are also the product of sexual reproduction (with a few exceptions), and thus provide genetic variability, which not only benefits natural populations but also the work of plant breeders.
annual plants use seed production as a form of dormancy. The seed will only germinate when conditions are again favorable, and the subsequent plant will grow, flower, and set seed, thereby complet- ing another cycle. seed banks around the world collect seed and keep it in a dormant state, usually at very low temperatures. some seed banks are concerned with the conservation of agricultural seed diversity (such the svalbard Global seed Vault on the island of spitsbergen in norway), and others with wild species (such as the Millennium seed bank Project in west sussex, england). such efforts could ensure the conservation of entire species or valuable crop plants in the event of a global catastrophe.
seed structure
any angiosperm seed consists of two essentials: an embryo and a seed coat (testa). a food store (endosperm) could also be included as an essential component, were it not for the fact that some highly specialized seeds have overcome the need for an endosperm (see p. 75).
The embryo consists of a plumule (the nascent stem), a radicle (first root), and either one or two seed leaves (depending on whether it is the seed of a monocot or a dicot).
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will only germinate in the presence of a suitable fungal counterpart, which forms a close association with the developing seed and provides its nutriment.
orchid seeds must represent an extreme of seed evolution; they have reduced their contents to the bare minimum so that their seeds are almost dust-like, with each plant producing an uncountable number each season. They are dispersed in the wind. The tiny, pin-prick seeds of vanilla (Vanilla planifolia) are an example.
The testa is the seed covering, enclosing its contents but perforated at one point called the micropyle. Its main function is to protect the embryo from physical damage and dessication. It can be thin and papery, as in peanuts (Arachis hypogaea), or very tough, as in the coconut (Cocos nucifera). The micropyle allows passage of oxygen and water upon germination. a scar, known as a hilum, is often present where the seed was once attached to the ovary wall.
some seed coats have extra features such as hairs (as in cotton, Gossypium), arils (such as the fleshy substance attached to individual pomegranate seeds, Punicum granatum), or fatty attachments known as elaiosomes. These often aid dispersal of the seed.
The endosperm is a mass of food-storing tissue, and its purpose is to supply the seedling that grows from the embryo as it germinates, as well as providing a source of energy during dormancy. The seeds of orchids (family Orchidaceae) have no endosperm and
B o t a n y i n a c t i o n
Dispersal of seeds
Plants that produce small seeds can produce many seeds, which is one strategy for ensuring that at least one of them will land in a favorable place. Those with large seeds produce fewer seeds, investing more resources and energy in each one. Their dispersal strategies are usually far more specific. The largest seed of all is the coco-de-mer (Lodoicea maldivica),
which can weigh up to 66 lb (30 kg).
Small seeds ripen faster and often spread further. Larger seeds may result in larger and stronger seedlings that can out-compete other plants. The strategies that plants adopt are huge and varied; it is impossible to say that one is better than another.
Punica granatum, pomegranate
dicotyledon seed
Testa (seed coat)
Monocotyledon seed Endosperm Cotyledons (two) Embryo Embryo Pericarp fused
“ th e l a r g e s t r i v e r i n t h e w o r l d r u n s t h r o u g h
t h e l a r g e s t f o r e s t. fa n c y—t w o m i l l i o n s o f
s q ua r e m i l e s o f f o r e s t, u n i n t e r r u p t e d s av e b y
t h e s t r e a m s t h at t r av e r s e i t.”
Richard Spruce
r
ichard spruce was a great 19th-century botanical explorer, spending 15 years exploring the amazon river, from the andes to its mouth. he was one of the first europeans to visit many of the places along the river.spruce was born in yorkshire, england. from an early age, he developed a great love of nature and natural history, and making lists of plants was among his favorite pasttimes. at the age of 16 he drew up a list of all the plants he had found in the area where he lived. These were arranged alphabetically and contained 403 species, a task that must have
taken some time and obviously a labor of love for one so young. Three years later he had drawn up the List of the Flora of the Malton District, containing 485 species of flowering plants, many of which are mentioned in henry baines’s Flora of Yorkshire (1840).
he became particularly interested in bryophytes—mosses and liverworts—and was an acknowledged expert, with a sizeable herbarium of his own comprising specimens from the
british Isles and farther afield. This early interest in plants led him to carry out a major expedition to the Pyrenees in 1845 and 1846. his intention was to finance his expedition by selling sets of flowering plant specimens, but the little-known bryophytes did not create much interest. during his collections from this region he discovered at least seventeen species new to science, and increased the bryophyte list of the area from 169
species to 478.
two years later, he was approached by william hooker, director of the royal botanic Gardens, Kew, to carry out the botanical exploration of the amazon on Kew’s behalf. despite failing health, he agreed, as it was a huge opportunity for him.
R i c h a r d S p r u c e
1 8 1 7 – 1 8 9 3
richard spruce amassed a huge collection of plants and other objects from his travels.
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Cinchona pubescens, quinine
richard spruce’s collection of quinine bark helped millions fight against malaria.
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again, he financed the trip by selling sets of specimens to interested naturalists and institutions in europe.
during the following fifteen years his travels took him along the amazon river to brazil, Venezuela, Peru and ecuador, where he collected more than 3,000 specimens, making him a major contributor to the knowledge of the flora of the area. spruce was also a keen anthropologist and linguist, learning twenty-one different languages while he was there, and collected many locally produced items of ethnobotanical, economic, and medical interest as well as plants.
he discovered Banisteriopsis caapi and observed its use among the tukanoan Indians of brazil. This is one of the two ingredients in ayahuasca, a brew of psychoactive ingredients used by the shamans of the indigenous western amazonian tribes in religious and healing ceremonies.
of the thousands of plants that spruce collected, the most important were undoubtedly Cinchona, or Quina, from ecuador, a genus in the family Rubiaceae from which quinine bark was harvested. native south americans used the bark as treatment for malaria. spruce provided seeds of the trees to the british government, making bitter bark quinine widely available for the first time, and so enabling the establishment of plantations in british colonies across the world and helping millions fight against malaria. after returning to england, he wrote The Hepaticae of the Amazon and the Andes of Peru and Ecuador.
other published work of his includes descriptions of twenty-three new british mosses, about half of which he had discovered, in the London Journal of Botany, and his “List of the Musci and hepaticae of yorkshire” in The Phytologist; in this he recorded forty-eight mosses new to the english flora and thirty-three others new to that of yorkshire.
spruce received a doctorate from the academia Germanica naturae curiosorum (German academy for those who are curious about nature) in 1864, and later became a fellow of the royal Geographical society.
The plants and other objects collected by spruce form an important botanical, historical, and ethnological resource. The richard spruce project is a joint initiative between royal botanical Gardens, Kew and The natural history Museum, comprising specimen location and databasing, specimen imaging and transcription and imaging of spruce’s original notebooks. More than 6,000 specimens have been completely databased and imaged, making information available to botanists, historians, and others interested in the exploration of the amazon and andes.
Sprucea (now Simira, in the family Rubiaceae), and the liverwort Sprucella are named after him. The standard author abbreviation spruce is used to indicate him as the author when citing botanical names of plants.
richard spruce was particularly interested in bryophytes— mosses and liverworts—from an early age and became an acknowledged expert.
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