The seeds of a plant are one of the most important structures involved in the growth of new plants. The emergence of a plant is dependent upon the successful germination of the seed within the soil. The zygote undergoes mitotic divisions to form an embryo which, under the right conditions has the potential to grow into a complete plant. A typical seed consists of three parts- the seed coat, the embryo and the endosperm. The embryo has the potential to develop into a complete plant, under suitable conditions. A mature seed’s internal structure typically comprises three parts- the embryonic root (i.e., the radicle), the embryonic shoot (i.e., the plumule), and the food reserves (i.e., the cotyledons). Let us explore the characteristics and functions of embryonic roots and shoots in more detail.
The radicle is that part of the embryo of the seed, which eventually develops into the root system of the plant. It is the first structure of a plant that arises in the plant embryo. The tip of this initial root is covered by a few loosely arranged cells, which form the root cap.
The seedling, under the right conditions gives rise to a new plant. The radicle is a positively geotropic structure that grows into the ground. Depending upon whether the seed is dicotyledonous or monocotyledonous, the radicle may develop either into a taproot system, wherein lateral roots grow from the main root; or, it may develop into a fibrous root, respectively.
The radicle is the basal tip of the hypocotyl, i.e., the structure below the cotyledons or the seed leaves
The radicle is the first structure that emerges from a sprouting seed
A root cap covers the tip of the radicle, which protects the rudimentary radicle and the mature root from environmental stresses, as the radicle pushes its way through the soil
The radicle mostly contains meristematic tissues which help in the division and growth of the root
It is a positively geotropic structure, that grows towards the ground in response to gravity
An essential feature of the root radicle is its positive hydrotropism, i.e., the tendency of the root to grow in the direction of a moisture gradient
The radicle is negatively phototropic
The radicle emerges from a structure in the seed known as the micropyle or the seed pore, which allows water to enter the seed. The emergence of the radical is crucial for the growth and development of the embryonic leaves, i.e., the cotyledons. The radicle and the hypocotyl elongate as the plant gets ample nourishment and water. This is what leads to the germination of a seed, i.e., the emergence of the radicle or the sprouting of a seed. Once the seeds germinate into a seedling, the seedlings are often transplanted onto larger fields.
The plumule is the initial shoot of the embryo that forms above the epicotyl of the embryonic axis. Once the plant germinates, the plumule becomes the shoot of the plant, as a result of the extension of the embryonal axis.
The plumule is the tip of the epicotyl, which is the structure above the cotyledons
The plumule is the second structure that emerges from a sprouting seed, following the radicle
The plumule consists of the leaf primordia and the meristem
The plumule eventually develops into the shoot of the plant which bears full-grown leaves that perform photosynthesis for the plant
The plumule is a negatively geotropic structure which grows above the ground
The plumule is neutral to the presence of water gradients
Additionally, the shoot of the seedling is positively phototropic, since it grows towards sunlight. This is essential in order to allow the young plant to perform photosynthesis.
The plumule develops into the stem of the plant after the seed germinates. It also bears the initial leaf primordia which gives rise to the leaves of the plant. As expected, the plumule is the initial structure that ultimately grows into the photosynthetic portion of the plant. It synthesises glucose for the plant and helps facilitate the growth of the seedling into a mature, fully grown plant.
Although they eventually develop into different structures, the radicle and the plumule share some similarities too −
Both are embryonic structures of the seed and are indispensable elements of the complete development of a plant.
They are mainly comprised of meristematic tissues.
Both structures are joined to the cotyledons and their growth is dependent on external and internal factors.
| Radicle | Plumule |
|---|---|
| The radicle is the embryonic root of a plant | The plumule is the embryonic shoot of a plant |
| The radicle is the first part that emerges from the seedling | The plumule emerges after the radicle |
| The radicle emerges at the tip of the hypocotyl | The plumule emerges at the tip of the epicotyl |
| The radicle grows toward the ground, i.e., it exhibits positive geotropism | The plumule grows above the ground, i.e., it exhibits negative geotropism |
| The radicle is positively hydrotropic | The shoot is neutral in terms of hydrotropism |
| The radicle terminates the hypocotyl | The plumule terminates the epicotyl |
The radicle and the plumule are the rudimentary root and shoot of a developing plant, found in the embryo within the seed
The radicle, present at the base of the embryonic axis, emerges from the micropyle and grows to give rise to the root system of the plant
The function of the radicle is to supply the developing seedling with nutrients and ample water so that it may grow into a fully mature plant
The radicle exhibits positive geotropism and hydrotropism and negative phototropism
The plumule, present at the tip of the embryonal axis, emerges after the radicle and develops into the shoot system
The shoot exhibits positive phototropism and negative geotropism
Q1. What is meant by epigeal germination and hypogeal germination?
Ans. Epigeal germination is said to occur when the cotyledons of the seed emerge out from the ground and acquire the ability to perform photosynthesis. Hypogeal germination occurs when the cotyledons remain beneath the soil
Q2. What is the difference between a monocot's radicle and plumule and that of a dicot?
Ans. In monocots, the radicle is covered with a sheath known as the coleorhiza, while the shoot is covered with a sheath called the coleoptile.
Q3. What are some of the hormones that promote seed germination?
Ans. Hormones such as gibberellins, cytokinins and ethylene are known to promote the emergence of the radicle from the seed
Q4. Which hormones are involved in breaking the dormancy of a seed?
Ans. Abscisic Acid (ABA) is involved in maintaining the dormancy of a seed, restricting its germination.
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