The seeds of groundnut differ in size, shape and colour of the seed coat or testa. The testa is thin and papery. In general, 3 unicellular layers viz., the outer epidermis or sclerenchyma, middle parenchyma and inner parenchyma constitute the testa. These layers are maternal tissue representing the integument of the maturing ovule.

• Seed size is an important economic character. Seed length ranges from 7 to 21 mm and seed diameter from 5 to 13 mm. Seed weight is also an important distinguishing character, which ranges from 0.17 to 1.24 g.
• Colour of seed coat or testa is an important criteria for classifying cultivated groundnuts and this may also influence the marketability of a cultivar. The colour characteristic is highly subjective and the different grades are difficult to describe on a uniform basis. However, this is an important diagnostic genetic character.
• Each seed consists of 2 cotyledons, upper stem axis and young leaf primordia (epicotyl) and lower stem axis (hypocotyl) and primary root. The embryo of the seed is straight rather than curved. The embryo contains all the leaves and the above ground parts that appear during the first two weeks of growth.
• The epicotyl consists of 3 buds - 1 terminal and 2 cotyledonary laterals. The former has 4 and the later has 1 or 2 leaf primordia. Thus the dormant embryo has 6 to 8 differentiated leaves ready to expand immediately after emergence.

Groundnut is a herbaceous annual with a fairly developed root system and a tap root. Tap root appears on the second day after seed germination and has a massive root cap. It elongates rapidly and grows almost vertically. It may vary from a few millimeters in diameter in annual species to 10 cm in perennial species. The well developed tap root may penetrate to a depth of 130 cm but rarely goes beyond 90 cm. The root system is normally concentrated at a depth of 5 to 35 cm and root spread is confined to a radius of 12 to 14 cm.

• The root system of spreading types are usually more vigorous than the bunch types. The lateral roots appear on the third day after seed germination. They are basically similar to tap roots but they lack the central pith and they multiply very quickly (as many as 100-120) on the fifth day and grow to a length of 15-20 cm.

The young stems are angular, usually pubescent and solid with a large pith. As the plant grows, the stems become hollow and tend to be cylindrical and shed hairs.

• The thickness of stem is highly variable. Generally the bunch types have thicker stems. Internodes are short and highly condensed at the base but are longer at the higher nodes.

The leaves in the genus are tetrafoliate. Occasionally small and abnormal leaflets may appear. The leaves of cultivated species are paripinnate with two pairs of opposite, sub-sessible, obovate (variable), shortly by mucronate leaflets with entire ciliate margin.

• The leaflets are borne on a slender, grooved and jointed rachis. Groundnut cultivars differ in leaf characteristics such as leaf colour (foliage colour), shape, hairiness and size. Stomata appear on both sides of the leaf.

• The inflorescence of the groundnut appears as a cluster of flowers in the leaf axils and is a reduced monopodium, either simple or compound. It consists of three or more flowers, is spike-like and always occurs in the axils of cataphylls or foliage.


• Flowers are enclosed in between 2 bracts. One of them is simple, subtending a short peduncle and the other bifid, subtending the pedicel. The flower is sessile but appears stalked after the growth of a tubular hypanthium just before anthesis.


• The calyx has 5 lobes. The typical papilionoid corolla is inserted on the top of the hypanthium and surrounds the staminal column. The stamens are 10, monadelphous with the staminal column surrounding the ovary.


• The pistil consists of a single ovary surrounded by the base of the hypanthium. The stigma is club shaped or clavate, usually at anther level or protruding slightly above.

Fertilization is normally completed before midday. After that the flower droops, the corolla closes, the calyx tube bends and the flower withers. During the early development of the young embryo, the ovary at the base of the calyx tube becomes mobilized for growth within a week. By then an intercalary meristem below the ovary is activated. The green ovary turns purplish from the tip downwards. The developing ovary pierces through the floral parts by the activity of the meristem to reveal an elongating peg or carpophore. The peg is a stalk-like structure that bears the fertilized ovules at its tip. Its growth is positively geotropic, until it has penetrated the soil to some depth. The tip then becomes diageotropic.

• After some initial development, the ovary shows no apparent change until it is diageotropically positioned in the soil. It is then that the ovary starts developing into a fruit.

• The fruit of groundnut normally referred to as a pod, is a lomentiform carpel, indehiscent and up to 10 cm in length. The mature pod normally contains up to 4 seeds, depending on the cultivar. Single seeded pods may be produced when all the ovules but the proximal abort.
• The pod size may range up to 6.0 x 2.7 cm. The fruit consists of 2 valves, structurally dehiscent but functionally indehiscent. When pressed the pods split along the longitudinal suture. The tip of the indehiscent fruit may end in an appendage called the beak.

• The cultivated groundnut has a distinct main stem and a varying number of lateral branches. The carriage of laterals is an important character which determines the growth habit of the plant.
• Two distinct forms of growth habits have been reported in groundnut. They are, Spreading (runner, trailing, procumbent and prostrate) and Erect (upright, erect and bunch).
• The spreading form is usually characterized by an erect conspicuous main stem with procumbent or documbent lateral branches. In the erect types the main axis becomes indistinguishable from the laterals. An intermediate semi-spreading form (spreading bunch, bunch runner and runner bunch) is also reported.

• The two main botanical sections of Arachis hypogaea differ in the distribution of vegetative branches and inflorescence in the axils of the leaves on the main axis and the branches.
• The main branch (axis) of cultivated groundnuts is designated as `n' and the first, second and third branches are called n+1, n+2 and n+3 respectively. In all forms of the species, primary vegetative branches (n+1) arise on the axis of the cotyledons and at a number of higher nodes on the main axis.
• In the sequential type (bunch types) inflorescence are borne at a second and several subsequent nodes of the primary branches. The first node on such a branch may bear secondary branch (n+2) but often it to bears an inflorescence, so that the first flowers are initiated very soon after the development of n+1 branch.
• In the alternate branching type (spreading type), the first two nodes of n+1 branch normally bear vegetative branches (n+2), the next two bear inflorescence and the next two again vegetative branches and so son. The same sequence is repeated for n+2 branches.
• In the Spanish group of sequential type, the n+1 branches grow upwards from the very beginning whereas in the Valencia group, these branches grow outwards first and then upwards. These two groups are generally referred as bunch type.
• In the alternate branched section, the runner forms have prostrate n+1 branches, whereas spreading forms more upright branches, both constituting spreading types.
• The sequential and alternate branching types also differ in many other agronomic characters: the duration is 110-120 days in the sequential and 130-150 days in the alternate branched; seeds are dormant in the alternate branched and non-dormant in the sequential branched; and plants are light green in sequential and dark green in alternate branched.
• The production of leaves and increase in shoot weight are considered as a measure of vegetative growth. The period of maximum growth is between 56 and 97 days in bunch varieties and 70 and 125 days in spreading ones. Higher rate of growth during early stages and more dry matter accumulation was seen in bunch types in comparison with spreading types.
• The total number of leaves per plant showed an exponential increase from about 21 days to 90-100 days after sowing and it ranged from 93 to 112 in bunch varieties and from 206 to 346 in spreading types.
• Maximum Leaf Area Index reported was 4.0 and for maximum yield the leaf area index at the 14th leaf stage should be more than 4, the total plant dry matter more than 500 g/m2 and the leaf dry matter more than 175 g/m2.

• In bunch types flowering commenced from 26 to 34 days after sowing. First flower opened generally 7 to 10 days later in runner types than bunch types. Onset of flowering was gradual, flower production began to accelerate 3 weeks later in Virginia types and 2 weeks later in the bunch types. During the 5 weeks after the first flower appeared, 66% of the total flowers were produced in bunch types and 79% in Virginia types.
• The number of flowers produced per plant ranges from 40 to 250 in spreading types and 98 to 137 in bunch types. Fruiting efficiency depends on the pattern of flowering (number of flowers at different period of flowering), which is more important than total number of flowers per plant. The flower production increased rapidly immediately after the commencement and reached the peak in about a week, and later the number of flowers produced per day declined. Maintained a lower rate of flower production for about 10 days.
• A second spell of increased flower production of lesser intensity than first occurred, and finally, there was a gradual decrease until cessation, 75 days after sowing. The cyclic flowering is inherent in the developmental process of groundnut and is not directly controlled by variation in environmental factors. Flowering gets reduced as pegging and fruiting progress.
• Daily minimum mean temperature recorded 0-3 days after flowering were positively correlated with flower production. Maximum temperature had a negative effect on production of flowers and high light intensity reduced it.
• Flowering stopped when the soil moisture dropped to wilting point but continuation of fruiting depended on the length of the drought. Relative humidity had a positive effect on the daily production of flowers.

• The reproductive growth in groundnut occurs over a period of at least 2 months. The groundnut flower opens before 6.00 a.m. and fertilization completes before midday. The flower droops, the corolla closes and calyx tube bends down and the flower withers away in 3 days.
• The peg is visible in about 5-7 days. The peg enters the soil in 2-8 days after initiation of the gynophore development in the bunch types and in about 5-10 days in the semi-spreading and spreading types. Usually 5-6 days after the peg enters the soil, development of pod commences. The peg ceases elongation as soon as the pod begins to develop.
• The lower ovule develops first. The enlargement of the pod proceeds from the base to the apex. The basal seed develops early and abortion or injury to the apical seed results in one shelled nut. Complete development of pod takes about 60 days from the time of fertilization
• Under normal conditions with concurrent flowering, fruiting and vegetative growth, 30-50% of the flowers usually do not develop into fruits. The ovaries of pollinated flowers can remain dormant for several weeks without losing their ability to resume active fruit development.
• The fertilized flowers range from 49 to 58.9% in spreading types and 21.9 to 67.5% in bunch types. Generally, a large number of early formed flowers develop into fruits. Flowers that appear 70 days after flowering do not form pods and fail to increase the yield due to low yield of mature pods
• Pod set percentage (the ratio of the number of mature pods to the total number of flowers) ranges from 8 to 17%.

• Arachis hypogaea was first published as species by Linnaeus in 1753. The taxonomy of the genus is not well-delineated and additional unidentified taxa are being regularly reported.
• The wild species show marked interspecific variation in growth habit, means of propagation and various morphological features of both vegetative and generative structures. Both annual and perennial forms occur and in some cases this nature is difficult to ascertain.

• Arachis is a perennial or annual legume with 3 or 4 foliolate, stipulate leaves, papilionate flowers, a tubular hypanthium and underground fruits (pods). A structure unique to the genus is the `peg' which is an expanded intercalary meristem at the base of the basal ovule. The expansion results in a lomentiform carpel of 1to5 segments, each containing a single seed with two massive cotyledons and a straight embryo.

• At present there are 22 species have been described in Arachis. They are

A. batizocoi Krap. & Greg.

A. villosa Benth.

A. diogoi Hoehne

A. helodes Mart. ex Krap. & Rig.

A. hypogaea Linn.

A. nambyquarae Hoehne

A. monticola Krap. & Rig.

A. tuberosa Benth.

A. guaranitica Chod. & Hassl.

A. paraguariensis Chod. & Hassl.

A. benthamii Handro

A. martii Handro

A. rigonii Krap. & Greg.

A. repens Handro

A. burkartii Handro

A. glabrata Benth.

A. hagenbeckii Harms.

A. prostrata Benth.

A. marginata Gard.

A. villosulicarpa Hoehne

A. lutescens Krap. & Rig.

A. pusilla Benth.

A. hypogaea Linn.

• In the intra-specific classification, the cultivated groundnut is divided into two large botanical groups viz., Virginia and Spanish- Valencia on the basis of branching pattern. The presence or absence of reproductive nodes on the main axis and the arrangement of reproductive and vegetative nodes on the laterals are the most important criteria.
• The sub specific classification can be summarized as follows.

• habit procumbent, decumbent or erect; branching alternate; inflorescence simple or never borne directly on the main axis, first branch on the cotyledonary lateral always vegetative: 2 or 2-4 seeds per pod; pod beak pronounced, moderate or absent; pod constriction prominent, moderate or absent; pod very large (> 20 mm) or small ( < 10 mm); testa colour commonly tan but red, white, purple and variegated forms exist; seed dormancy usually present; foliage dark green.

• habit procumbent, decumbent or erect; main axis in procumbent forms short (not exceeding 40 - 50 cm); stem usually not very hairy; usually 2 seeded; medium to late maturing.

• habit procumbent; main axis may exceed 1 m; stem fairly hairy; pods strongly beaked with 2-4 seeds; very late maturing.

• habit erect to decumbent; branching sequential; inflorescence simple or compound, always present or main axis; first branches on cotyledonary laterals reproductive; seed dormancy usually absent; foliage usually lighter in colour than in sub sp. hypogaea

• habit procumbent, decumbent or erect; branching alternate; inflorescence simple or never borne directly on the main axis, first branch on the cotyledonary lateral always vegetative: 2 or 2-4 seeds per pod; pod beak pronounced, moderate or absent; pod constriction prominent, moderate or absent; pod very large (> 20 mm) or small ( < 10 mm); testa colour commonly tan but red, white, purple and variegated forms exist; seed dormancy usually present; foliage dark green.

• vegetative branches or primaries absent or regularly placed at the distal nodes; inflorescence usually simple; pods with 2 or 2-4 (rarely 5) seeds; beak absent, slight or prominent; size medium to small; testa colour tan, red, white, yellow, purple or variegated.

• vegetative branches occasional and irregularly placed; inflorescence compound; pods usually with 2 seeds; beak present or absent; size medium to small; testa colour tan, red, white or purple.