The evolution of plants has resulted in increasing levels of complexity, from the earliest algal mats, through bryophytes, lycopods, ferns to the complex gymnosperms and angiosperms of today. While the groups which appeared earlier continue to thrive, especially in the environments in which they evolved, each new grade of organisation has eventually become more "successful" than its predecessors by most measures.
1. Evidence suggests that an algal scum formed on the land 1,200 million years ago
2. To thrive and to avoid extinction, plant are made mechanisms and evolved seed plant during 200 million years ago
3. The latest major group of plants to evolve were the grasses, 40 million years ago
4. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years.
SEED DEVELOPMENT, MATURATION AND SEED STRUCTURE
A true seed is defined as a fertilized mature ovule consisting of embryo, stored food material and protective coats.
The important events involved in seed development and maturation include
3. Development of the fertilized ovule by cell division
4. Accumulation of reserve food material
5. Loss of moisture content.
The mature anthers dehisce and release pollen -grains (haploid microspores). When pollen grains are transferred from an anther to the stigma of the same flower the process is called self-pollination or autogamy. If they are transferred to the stigma of another flower, cross-pollination or allogamy is said to have occurred.
Self-pollination occurs in those plants where bisexual flowers achieve anther dehiscence and stigma receptivity simultaneously called as chasmogamy. The majority of angiosperms bear chasmogamous flowers.
In some plants, flowers do not open before pollination such flowers are called cleistogamous, and this is the most efficient floral adaptation for promoting self-pollination.
Cross-pollination is ensured in plants which bear unisexual flowers. In bisexual flowers also self-pollination may be prevented by
a. Self-sterility : inability to produce viable pollen Eg:Sunflower
b. Dichogamy: maturation of male and female organs at different times Eg:Bajra
c. Herkogamy : where the structure of male and female sex organs proves a barrier to self pollination Eg: Lucernae
d. Heterostyly :where flowers are of different types depending on the length of the style and stigma and pollination occurs only between 2 dissimilar types Eg: Brassica
e. Self incompatibility: Inability to viable pollen to fertilize ovule of same flower Eg: Cole crops
Self-pollinated crops: wheat, rice, barely, mungbean and cowpea
Cross pollinated Crops: Maize, rye, carrot, cauliflower and onion.
Often cross pollinated crops: cotton and pigeon pea where there may be 10-40 % cross pollination.
Agents bring about the dissemination of pollen grains
1. Abiotic : wind (anemophily) and water (hydrophily)
2. Biotic including insects (entomophily) and bats (cheiropterophily).
After landing on the stigma, the pollen grain germinates and pollen tube grows through the style. The surface of the stigma secretes substances, which may provide optimum conditions for pollen germination. The pollen tubes traversing the style pectinase which dissolves intercellular substances of the style tissue. After traversing the style, the pollen tube enters embryosac of the ovule. The embryosac consists of 8 cells. The end near the micropyle has the egg apparatus, which consists of egg cell and 2 synergids. There are 2 polar nuclei in the centre and the chalazal end has 3 antipodal cells.
In angiosperms, fertilization involves the participation of 2 male nuclei (double fertilization). One fuses with the egg nucleus to form the diploid zygote and the other with 2 polar nuclei to produce a triploid nucleus, which is the primary endosperm nucleus.
3. Seed Development
1 Embryo Development
The first division of the zygote is transverse in dicots and it results in a small apical cell and a large basal cell . Cell ca divides vertically forming 2 juxtaposed cells and cb undergoes a transverse division forming 2 superimposed cells. These results in a T-shaped, 4 celled proembryo.
Cell ci divides transversely giving rise to n and n'. These 2 cells divide further resulting in a row of 3 or 4 cells, forming suspensor.
Cell m and its derivatives undergo vertical divisions forming a group of 4 to 6 cells. This group divides by oblique-perclinal wall forming a set of inner cells and a row of outer cells. The inner cells form the initials of the root apex and the outer cells form the root cap.
The 2 cells formed as a result of the division of ca again divide vertically forming quadrant. Each cell of the quadrant divides transversely and thus an octant containing 2 tiers of cell l and p is formed.
The cells of the octant undergo vertical division resulting in a globular proembryo. Periclinal divisions occur in the peripheral cells of the globular proembryo that delimit an outer layer, the dermatogen. The tier l gives rise to cotyledons and shoot apex and l forms hypocotl-radicle axis.
Certain deviations from the above pattern of embryo development are found in different plants. Different types of embryogeny are distinguished depending on the plane of division of the apical and the extent of contribution of the basal cell towards embryo development (in some plants cb remains undivided and does not take part in embryo development at all).
In monocotyledons, the cell cb remains undivided and develops into a haustorial of the suspension. Cell ca divides into 2 by a transverse division. The terminal cell of these 2 by repeated divisions in different planes gives rise to a single cotyledon. The embryo development in grasses is different from that of other monocotyledons. A dorsiventral symmetry is established as a result of the peculiar oblique position of cell walls early in the embryogeny. The single cotyledon is reduced to absorptive scutellum and additional structures like coleptile and coleorrhiza are formed.
2 Endosperm Development
There are 3 types of endosperm development (a) nuclear - where the endosperm nucleus undergoes several divisions prior to cell wall formation, e.g., wheat apple, squash, (b) cellular -in which there is no free nuclear phase, and (c) helobial where the free nuclear division is preceded, and is followed by cellularization as in some monocots. During the course of seed development, reserve food materials are accumulated in the endosperm from the adjacent tissues.
In endospermic dicot seeds, endosperms are retained as a permanent storage tissue. In non-endospermic dicot seeds, endosperm reserves are depleted and occluded by the developing embryo. The reserves are then reorganized in the cotyledons, which in turn act as the source of stored reserved food for embryo after germination. A part of the endosperm is depleted in cereals during embryo maturation and this lies as a layer between the starchy endosperm and scutellum.
3 Seed-coat Development
Integument's of the ovule undergo marked reorganization and histological changes during maturation to form seed coats. In bitegmic ovules (which have 2 integument's), the seed coat may be derived from both the integument's or from the outer integument only; the inner integument may disintegrate.
4 Seed Structure and Functions
Seeds may be broadly classified as dicotyledons and monocotyledons, depending on the number of cotyledons. Dicotyledons seeds may be either non-endospermic (exalbuminous) e.g. chickpea, pea and bean or endospermic (albuminous) e.g., castorbean, fenugreek, etc., Monocotyledons seeds are mostly albuminous.
A typical non-endospermic dicot seed is made up of seed coat and embryo. The seed coat consists of 2 layers that may be united or free, the outer layer, which is hard and made of thick walled cells is called testa and the inner thin membranous layer is called tegument. The seed coat is of considerable importance because it is the only protective barrier for the embryo from the external environment.
1. The seed coat bears a scar called hilum, marking the point at which seed is attached to stalk.
2. The funicle or the stalk forms a ridge called raphe along the margin of the seed.
3. At one end of the hilum, there is a small hole called micropyle. There is an outgrowth below the hilum in leguminous seeds, which is called strophiole.
4. Certain other seeds (castorbean, nutmeg) have outgrowths called arials.
5. Arillar contents may important in attracting animals, which aid in seed dispersal.
The embryo consists of embyonic axis and 2 fleshy cotyledons. The axis includes embyonic root (radicle), hypocotyl to which 2 cotyledons are attached and plumule (shoot apex with first true leaves). The cotyledons of non-endospermic e.g., pea are bulky and account for over 90% of the mass of the seed.
Cotyledons of epigeal, non-endopermic species become leaf like and photosynthetic after germination. In endospermic dicot seeds, the endosperm is bulky and stores food reserves. In these cases, the cotyledons are small or haustorial in nature.
The nucleus of the ovary after fertilization becomes perisperm. The perisperm in the majority of seeds fails to pass beyond an incipient stage of development but in a few cases this tissue becomes the store for food reserves such as coffee.
Poaceae seeds are generally enclosed in one seeded fruit called caryopsis. The seed coat is fused with fruit wall to form pericarp. The endosperm forms the main bulk of the grain and is the tissue for food storage. It is separated from the embryo by a definite layer known as epithelium.
The outer most layer of the endosperm is the aleurone layer, which unlike the rest of the endosperm, is made up of living cells devoid of galactomannan reserves. This layer secretes alpha-amylase and proteolytic enzymes which hydrolyse reserves of endosperm.
The embryo is very small and lies in a groove at one end of the endosperm. It consist of a shield shaped cotyledon (Scutellum) and a short axis with plumule and radicle protected by root cap.
The plumule as a whole is surrounded by coleptile, a protective sheath, and similarly the radicle including the root cap is surrounded and protected by coleorrhiza.
Scutellum supplies growing embryo with food material absorbed from endosperm through epithelium. The initial synthesis of alpha-amylase and certain proteolases also occurs in scutellum.
4.2 Seed Growth and Maturation
Wheat and soybean representing monocots and dicots may illustrate the changes in the pattern of accumulation of reserve materials at different stages of seed maturation.
In wheat, the dry weight of the seed increases rapidly in about 35 days after anthesis. The water content of the grain is maximum between 14 and 21 days after anthesis, and then it declines rapidly. The amounts of reducing sugar and sucrose are high between 7 and 14 days and decline rapidly thereafter due to conversion to starch. Since in wheat, starch is the major reserve material of the seed, the pattern of starch accumulation is similar to that of dry matter accumulation.
The speed of germination is faster in wheat varieties that begin to lose water early during seed development. The seed is said to have physiologically matured only when it attains maximum dry weight, germinability and vigour. Normally the seed is harvested at field maturity, a stage when the moisture content is reduced to about 6-10 % in wheat. Field maturity is a crop specific character.
A soybean seed attains maximum dry weight between 48 and 54 days after flowering. Oil accumulation is less during 12-18 days after fertilization; maximum oil accumulates between 24 and 42 days after flowering, after which the rate decreases. The protein content in the seed is maximum during 12-18 days after fertilization and decreases subsequently. The initial high percentage of protein may be due to the high content of non-protein nitrogen, which decreases with seed.
Components of Seed
It is the outer covering of seed and gives protection. It develops from the 2 integuments of ovule. Outer layer of the seed coat which is smooth and rough is known as the testa and is formed from the outer integument. The inner layer of the seed coat is called the tegmen and is formed from inner integument.
It is the mature ovule consisting of an embryonic plant together with a store of food, all surrounded by a protective coat, which gives rise to a plant similar to that of its mother. It is a miniature plant consists of plumule, radicle and cotyledon. The plumule and radical without the cotyledon is known as primary axis.
Rudimentary root of a plant compressed in the embryo is the radicle, which forms the primary root of the young seedling. It is enclosed in a protective cover known as coleorhiza.
It is the first terminal bud of the plant compressed in the embryo and it gives rise to the first vegetative shoot of the plant. It is enclosed in a protective cover known as coleoptile.
Cotyledons are the compressed seed leaves. A single cotyledon (Scutellum) is present in monocots while two cotyledons are present in dicots, hence they are named as monocots and dicots, respectively. In dicots they serve as storage tissue and are well developed, while scutellum is a very tiny structure in monocots.
Endosperm develops from the endosperm nuclei which is formed by the two polar nuclei and one sperm nuclei. It stores food for the developing embryo.
Appendages of seeds
Some seeds will have appentages that are attached to the seed coat. They vary with kind of seed. The appendages sometimes help in dispersal of seeds or in identification of genotypes. Some of the appendages are Awn, Hilum, Caruncle, Aril, Hair and Wings.
Awn : The thorn like projection at tip of the seeds. (eg) Paddy - The bract tip was elongated into the awn.
Hilum : It is the scar mostly white in colour present on the lateral side of the seed. It represents attachment of the seed stalk to placenta of the fruit to mother plant (eg) Pulses.
Micropyle : The point where the integuments meet at the nucellar apex has been referred as micropyle.
Chalaza : At region of integumentary origin and attachment opposite to micropyle is called chalaza.
Rapha : The area between the micropyle and chalaza is the rapha. The rapha may be visible on the seed coat of some species.
Caruncle: It is the white spongy outgrowth of the micropyle seen in some species (eg) Castor, Tapioca.
Aril : It is the coloured flesh mass present on the outside of the seed (eg) Nutmeg.
Hairs : They are the minute thread like appendages present on the surface of the seed (eg) Cotton.
Wings: It is the papery structure attached to the side of the seed coat either to a specific side of the seed coat or to all sides (eg) Moringa.