Anyone who watches common beans and peas emerge from the soil will notice a clear difference. The common bean lifts the leaves formed within the seed above the soil surface. In peas, these leaves remain below the soil surface while the young shoot grows upwards.
Germination types and environmental factors: how seeds germinate successfully
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These differences help to identify young plants after sowing and determine whether their emergence follows the typical development of the crop. This article explains what initially happens inside the seed, how the first leaves develop and why beans, peas and cereals emerge from the soil in different ways.
Table of contents
What germination types describe
Germination types describe different forms of early plant development. One important characteristic is the position of the cotyledons: these are the first leaf structures, which are already formed within the seed. In some plants, they are lifted above the soil surface, while in others they remain below it.
Which of these forms of development a plant follows depends on its species. Sowing conditions do not change this basic form. However, they do affect how quickly and vigorously a seedling grows, how its young plant parts elongate and whether it successfully reaches the soil surface.
What happens during seed germination?
For a viable seed to begin growing, it needs water, oxygen and a temperature suitable for the plant species. In some species, light or darkness is also an important germination trigger. As the seed takes up water, its metabolism becomes active and stored food reserves become available for growth.
In most cases, the radicle, the embryonic root, is the first part to emerge from the seed coat. It grows into the soil, anchors the young plant and begins to take up water and minerals. The shoot then develops.
In the narrower botanical sense, germination usually ends once the radicle has emerged. The seedling then continues to develop below the soil surface. Only when its shoot reaches the soil surface is this referred to as seedling emergence. A bed can therefore still look empty even though the first seeds have already germinated.
Until enough green leaves have formed, the seedling relies mainly on the reserves stored in the seed. As further leaves develop, photosynthesis becomes increasingly important: using light, the plant increasingly produces the substances it needs for growth itself.
The role of water, oxygen, temperature and light in the start of germination is explained in the article Environmental factors in seed germination: water, oxygen, temperature and light. How seeds are classified according to their response to light and cold is covered in the article Germination types and environmental factors: light, dark and cold germinators.
What are cotyledons?
Cotyledons, botanically known as cotyledons, are already part of the young plant within the seed. Their functions vary depending on the plant species. In common beans and peas, they contain a large proportion of the food reserves on which the seedling initially relies. In other plants, these reserves are stored mainly in the endosperm, a nutrient-rich tissue within the seed.
When cotyledons are lifted above the soil surface, they can also carry out photosynthesis once they turn green. They support the young plant while its true leaves develop. Later, many cotyledons lose their function. In many cultivated plants, yellowing, wilting and eventually falling off is part of normal development and, on its own, is not a sign of disease.
Epigeal and hypogeal germination
In many cultivated plants, seedling emergence shows whether the cotyledons are carried above the soil surface with the shoot or remain below ground. These two forms of development are known as epigeal and hypogeal germination.
Epigeal germination
In epigeal germination, the cotyledons are lifted above the soil surface. This is mainly achieved by elongation of the hypocotyl, the section of the shoot below the cotyledons. In many cultivated plants, the raised cotyledons emerge from the seed coat and unfold in the light. The first true leaves then become visible.
This is how common beans, tomatoes, peppers, sunflowers, radishes and many brassicas, among others, emerge from the soil. In common beans, the process is easy to observe: first, the radicle emerges. The elongating shoot then lifts the two thick cotyledons above the soil surface.
Hypogeal germination
In hypogeal germination, the cotyledons remain below the soil surface. The epicotyl, the section of the shoot above the cotyledons, grows upwards. The young shoot emerges above ground, while the cotyledons remain in the soil and continue to help supply it with nutrients. Whether they leave the seed coat or remain inside it is a separate characteristic.
Peas, broad beans, chickpeas and lentils develop in this way. Despite their similar names, common beans and broad beans therefore show different germination types: common beans lift their cotyledons above the soil surface, while broad beans keep them below it.
Special feature of grasses
In cereals such as maize, wheat and barley, the seed has a different structure. Grasses have only one cotyledon. It is called the scutellum, remains inside the seed during germination and transfers nutrients from the endosperm to the growing seedling.
In barley, the coleoptile is the first structure to push out of the grain towards the soil surface. This narrow protective sheath surrounds the first true leaf and protects it as it grows through the soil. Once its tip reaches the surface, the leaf emerges from it. This is why emerging barley looks different from a young bean or pea.
Epigeal and hypogeal germination compared
The two forms of development differ mainly in which section of the shoot elongates and where the cotyledons are positioned afterwards:
| Characteristic | Epigeal germination | Hypogeal germination |
|---|---|---|
| Position of the cotyledons | above the soil surface | below the soil surface |
| Elongation of the shoot | Hypocotyl, below the cotyledons | Epicotyl, above the cotyledons |
| Seedling emergence | The shoot lifts the cotyledons with it | The shoot grows upwards without the cotyledons |
| Example | Common bean | Pea |
This classification describes how the seedling emerges from the soil. On its own, it indicates neither how quickly a species germinates nor how resilient its young plants are in general.
Distinguishing cotyledons from true leaves
In plants with cotyledons visible above the soil surface, it is worth looking at the leaves that develop afterwards. Cotyledons are often simpler in shape and look different from the later foliage. True leaves grow from the young shoot. They often already show characteristics of the particular plant species and, as they grow larger, take on an increasing share of photosynthesis.
In tomatoes, the difference is easy to see: their cotyledons are smooth and relatively simple in shape. The true leaves that follow, by contrast, develop the more deeply divided form typical of tomatoes. In peas, the cotyledons remain below the soil surface, so the young shoot with its true leaves is visible above ground.
Distinguishing between the two helps identify a plant’s stage of development. This is particularly useful when several crops have been sown next to each other and their young plants still look unfamiliar. Leaf colour, growth habit and the development of further leaves then provide additional clues about how a seedling is developing.
Importance of germination type in gardening practice
The germination type helps identify newly emerged crops and assess problems with seedling emergence. In peas, a young shoot without visible cotyledons is normal. Common beans, by contrast, lift their cotyledons above the soil surface. If a hard soil crust forms after rain, it can slow this process and damage the curved shoot. If emergence is patchy, it is therefore also worth checking the soil surface. Such a crust can also hinder other seedlings whose cotyledons remain below the soil surface.
Good seedling emergence also depends on moisture, soil structure and a sowing depth suited to the crop. Germination type alone does not determine a fixed sowing depth. How plant species, seed size, light requirements and soil interact when choosing the correct depth is explained in the article Optimal sowing depth: the key to strong plants.
Further terms relating to seedling development
The terms epigeal and hypogeal describe the position of the cotyledons in relation to the soil surface. A separate question is whether they leave the seed coat. If the cotyledons emerge from it, the development is described as phanerocotylar. If they remain enclosed within it during further seedling development, it is described as cryptocotylar.
The two classifications describe different characteristics and can be combined. Even with epigeal development, the cotyledons can be lifted above the soil surface while still enclosed within the seed coat. For basic observation in the garden, their position in relation to the soil surface is the main distinguishing feature.
In summary
Germination is only the beginning: roots and shoots start developing before a seedling becomes visible. During seedling emergence, each crop shows its typical germination type. Whether the cotyledons are lifted above the soil surface or remain below it is part of this development and, on its own, says nothing about germination success.
Knowing these differences makes it easier to identify young cultivated plants after sowing and assess their stage of development. Whether they reach the soil surface also depends on sowing depth and soil condition. Adequate moisture and a loose soil surface support seedlings as they emerge.
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