Lecture 25

The Seed

I. Concept and Morphology

A. Definition: a seed is a ripened ovule that contains the embryo (young sporophyte), endosperm and is usually enclosed by a testa (seed coat).

B. The seed tissues are the products of double fertilization as well as parts of the previous (maternal) generation, e.g. the testa.

C. Seeds can survive longer than spores. This is offered as one reason for the dominance of angiosperms over free-sporing plants.

D. Seeds are economically extremely important. Grains and legumes provide food, beverages, fiber, oils, etc.

E. Storage tissues

1. Endosperm – formed from fused sperm and polar nuclei. Can be 3N, 5N, etc.
2. Perisperm – formed from nucellar tissue (materal sporophytic). Perisperm is found in the seeds of Caryophyllales. Example: Agrostemma githago (Caryophyllaceae) - image of seed in section.
3. Cotyledons. In legumes (Fabaceae), the cotyledons are the main storage organ; the endosperm forms but is absorbed by the cotyledons (Figure 23.1). Image of Arachis hypogaea (peanut).

F. Seed external features.

1. Hilum = scar left by the abscission of the funiculus from the seed.

Image of Abrus (Fabaceae). Is the position of the hilum important? Helps differentiate the harmless Rhynchosia from the poisonous Abrus (image).
The shape of the hilum varies as well, such as this linear one (image) in Strophostyles.
The hilum (Figure 23.9C) functions as a one-way valve. It allows moisture out (opens when the air is dry) but does not let moisture in. This is how legume seeds can attain very dry interiors.

2. Raphe. For anatropous ovules, the funiculus is adnate to the integument. Upon forming a seed, a ridge remains called the raphe.

G. Ovular outgrowths include:

1. aril – outgrowth from the funiculus. Examples: Magnolia (Magnoliaceae), Myristica (Myristicaceae), Pithecellobium (Fabaceae), Mischocarpus (Sapindaceae), Strelitzia (Strelitziaceae), Guiacum (Zygophyllaceae),
2. caruncle – integumentary protruberance near the micropyle. May aid in water absorbance or possibly dispersal. Example: Ricinus (Euphorbiaceae), Jatropha scaposa & J. curcas; J. subaequiloba (Euphorbiaceae).
3. elaiosome – an oily appendage that aids in ant dispersal. Seen in several Papaveraceae such as Sanguinaria (image) and Stylophorum (image1 of seed dry and image2 of seed fresh).

H. Classification of seeds by Martin (1946!) looked at 1287 genera of gymnosperms and angiosperms. Figure 23.2 emphasizes the size and position of the embryo.

II. Seed Development

A. Pisum (pea) example. Figure 23.3, Figure 23.4. Endosperm is first liquid (non-cellular), increases in size at first, like the embryo, then begins decreasing. Reason? It is being used up by the developing embryo. Mostly cell divisions are increase the ovule volume, later cell enlargement.

B. Growth of fruit is associated with increase in growth regulators (hormones) such as auxin, cytokinin, and gibberellins. Seeds are the source of these. Fertilization initiates growth of ovules, and that stimulates further hormone synthesis.

C. During embryogenesis, food is being stored in endosperm or cotyledons. Upon germination, there is a complete role reversal. The stored food is utilized by the growing embryo.

III. Seed Coat (Testa)

A. Seed coats vary in structure based upon the number of integuments, their thickness, vascularization, and developmental changes during seed maturation. Most testas are dry but some are more “juicy” such as Punica granata (pomegranate, Lythraceae - image) and some gymnosperms.

B. The testa inhibits germination by preventing the absorption of water and oxygen. Accomplished by having a thick cuticle and phenolic compounds, as well as mechanical resistance. The seed coats of legumes are famously hard and impervious. Examples: Amorpha (image), Cassia and Albizzia (image), Chamaecrista (image), and Dalea (image).

C. Germination ensues when dormancy is broken, which may require the removal of the testa (or part of it – scarification) or the leaching out of inhibitors. Borrowing a quote from J. Silvertown’s book review (1998, AJB 86: 903) “Seeds can exhibit notoriously idiosyncratic germination behavior, sometimes varying as much among the progeny of a single maternal parent as they do among different species.” For those interested in knowing more about this subject, required reading is the following:

Baskin, C. C. and J. M. Baskin. 1998. Seeds: ecology, biogeography, and evolution of dormancy and germination. Academic Press. 666 pp.

D. Seed coats and dispersal.

1. Animal dispersed seeds. Often the testa is hard and thick, thus requiring softening by passing through the acid environment found in an animal’s digestive system.
2. Sticky seeds. Many of the hooks and barbs found on fruits that are animal dispersed derive from the gynoecium (not the seed). But in some cases the seed itself is actually sticky.

Brassicaceae. The common peppergrass, Lepidium virginicum (images), has a layer of mucilage around its seeds.
Viscaceae such as Phoradendron (image of seed) and Arceuthobium (image of seeds). The viscin strands themselves are cellulosic but are surrounded by a matrix of mucopolysaccharides.

E. Examples demonstrating seed anatomy

1. Ricinus communis (castor bean, Euphorbiaceae), image of seeds. Figure 23.5. Ovule is bitegmic, crassinucellate (means with a large nucellus containing the embryo sac). An obdurator forms from the placenta that protrudes into the micropyle. This is later pushed away by the caruncle which forms near the micropyle. The embryo sac grows, crushes the nucellus. Eventually the embryo and endosperm fill most of the volume of the seed (Figure 23.5D). The outer and inner integuments change a lot, becoming sclerified in the process of becoming the testa.

2. Brassica and Sinapis (cabbage, mustard, etc., Brassicaceae). Ovule is bitegmic with thick integuments. The epidermal cells of the outer integument fills with mucilage. These cells absorb water and burst, leaving a mucilage coating on seed surface. The inner epidermis of the outer integument forms a palisade layer with lignified thickenings (Figure 23.6). The inner integument dies, may become a pigment layer.

3. Cucurbita (melons and gourds, Cucurbitaceae). Ovule is bitegmic, crassinucellate, but the inner integument dies early, so the testa forms from the outer one (Figure 23.7). The outer integument differentiates into several layers. Figure 23.8A, image1, image2, image3. Note epidermis, lignified hypodermis, sclerenchyma layer, parenchyma (aerenchyma), collenchyma, nucellus, endosperm, and cotyledon.

4. Legumes (Fabaceae - image of seeds), Glycine (soybean) and Phaseolus (bean). Ovule is bitegmic, the inner integument dies, and the outer one differentiates into many layers. Epidermis differentiates into a palisade of sclereids, subepidermal parenchyma, endosperm, cotyledon (Figure 23.8B, Figure 23.9, Figure 23.10). The palisade sclerenchyma is described a number of ways for the different appearances of the cells: columnar cells, pillar cells, hourglass cells, osteosclereids, lagenosclereids, macrosclereids. Images of seed anatomy of Phaseolus: image1, image2, image3, image4, image5.

IV. Nutrient Storage Tissue

A. Storage in endosperm or perisperm = albuminous seeds. Versus not storing in these but in cotyledons = exalbuminous seeds.

B. Endosperm formation.

1. Nuclear. Many nuclei formed by free-nuclear divisions. This is where karyokinesis occurs but no cell walls are formed around the products of division. Cell walls may or may not form later (Figure 23.11).
2. Cellular. Cell divisions occur with the first mitosis and continue. Found in magnoliids and four monocot families.
3. Helobial. The embryo sac is divided into two unequal cells. The larger (micropylar) one is noncellular – undergoes many free-nuclear divisions. The smaller (chalazal) one shows varying behavior. This type is seen in monocots.

C. Behavior of nucellar cells and endosperm nuclei of the embryo sac.

1. Capsella bursa-pastoris (Brassicaceae), images. Endosperm nuclei accumulate at two ends of embryo sac; ones at chalazal end digest nucellar cells.
2. Eranthis hyemalis (Ranunculaceae), images. Nuclear endosperm incorporates adjacent integumentary cells (basically, eats them up!) reducing the number of cell layers from 11 to 7.
3. Zea mays (Poaceae). Figure 23.11. Free nuclei occur along the periphery of the embryo sac. Cell walls first start to form around the nuclei positioned at the micropylar end. By four days after pollination the endosperm is all cellular. The outermost layer becomes the aleurone.

D. Variations

1. Cocos nucifera (coconut, Arecaceae). The central part of the embryo sac does not become cellular. Endosperm remains liquid = called coconut milk inside seed. The embryo is embedded in the outer, solid endosperm (image).
2. Endosperm haustoria. Some plants have these outgrowths from the chalzal or micropylar end of the ovule that “aggressively” invades surrounding tissues, drawing nutrients from them for the developing seed. Seen in plants such as Nemophila (Boraginaceae – see Berg 2009).
3. Ruminate endosperm (ridged and furrowed). Seen often in magnoliid families, such as Annonaceae (Asimina) and Myristicaceae (Horsfieldia, Knema).

Reference:

Berg, R. Y. 2009. Embryo sac, endosperm, and seed of Nemophila (Boraginaceae) relative to taxonomy, with a remark on embryogeny in Pholistoma. Amer. J. Bot. 96:565-579. Online paper HERE.

Last updated: 14-Oct-22 / dln