Seeds and Meristems
Inside each seed is a small plant, living and waiting for conditions to initiate sprouting. Each seed usually includes a pair of growing tips known as apical meristems. Grubbycup has the details on why meristems are so important throughout the life cycle of the plant, from germination to flowering.
Seeds are amazing storage containers that allow a plant to travel through space and time. Inside each protective casing is a tiny plant held in stasis until it is exposed to conditions that initiate development and growth, along with some stored energy (endosperm) for the plant to get started with.
The miniature plant inside often has either one seed leaf or a pair of seed leaves. Along with the seed leaf (or leaves), the miniature plant inside the seed shell also usually has a pair of growing tips (apical meristems), one at the top of the stem (hypocotyl) that develops into the first true leaves at the top of the sprout, and one at the tip of the root (radicle) used for root development.
Meristem cells are unusual in that they are a basic building block cell that can differentiate to become different types of cells depending on need and environment. Meristem cells can divide and generate new cells, an ability that becomes lost when they differentiate into the type of plant cell they ultimately become.
Stems, leaves, roots and flowers all start as undifferentiated meristem cells that have differentiated into the different cells needed to form the plant parts and organs. Although meristem cells can transform into different cells, once a cell type is determined, it loses the ability to become one of the other types.
In a nutshell (pun intended), a seed coat protects a tiny plant that already has a root, stem and a leaf or two. The metabolism of the plant is slowed dramatically, and dehydration stalls development. The embryonic plant is well developed and alive in a viable seed. If the plant dies, or is fatally malformed, then the seed is dead and will not sprout. Some varieties of plant seeds remain viable for years; others are best used the following season.
In general, moisture is bad for seeds unless they are being sprouted. Moisture can enter into the seed through small holes in the seed coat called micropyles where it can stagnate and encourage anaerobic bacterial and fungal growth. Most seeds prefer to be stored in a dry, cool location until used.
Under proper environmental conditions, moisture entering the micropyles will induce sprouting. If the micropyles are deformed or blocked, germination may be difficult. To assist in sprouting such seeds, they may be soaked in water before planting, or a small nick may be made in the seed coating (scarification).
A small cut opposite the hinge side of the seed can be made to allow moisture to be absorbed. An alternative to cutting the seed coat is to weaken it by scraping the seed against a bit of sandpaper or emery board.
As the dried plant takes up the water, it starts to grow. Since the tiny plant is no longer in stasis, it begins to need warmth, moisture and air. In fact, it becomes dependent on them. One way to reduce the number of weeds in a garden plot that relies on irrigation is to water an area so the weeds sprout, and then kill off the tender sprouts by allowing the area to dry out. Once seeds have started to sprout, they must either grow or perish.
As the tiny sprout grows, it bursts through the seed coat. The radius spreads underground to become the root system. The seedling leaves will unfurl, the seed coat will drop off and the shoot apical meristem (growth tip) will develop the first true leaves and begin normal growth.
Shoot meristems are usually indeterminate and will continue to grow as long as conditions permit and flowering is not triggered. Primary meristem cells add to the height of the plant and the length of the roots, while secondary (lateral) meristem cells add to the girth of the trunk, branches and roots.
Many plants have the ability to not only produce a main apical meristem, but also develop lateral growth tips. Plants that exhibit apical meristem dominance will produce a dominant shoot off the trunk that will inhibit the development of the side branches. By removing or pinching off the main apical meristem, lateral growth is encouraged. This is often done to encourage bushier growth.
Catnip is an example of a plant that responds well to pinching. While catnip's normal growth habit is a long shoot with a large floral cluster at the top, if pinched back, lateral growth is encouraged, and it will produce several smaller clusters, making for a bushier and more attractive plant.
Another important use of the apical meristem is in vegetative propagation (asexual reproduction). In many plants that have multiple growing tip nodes, undifferentiated meristem cells in the stem can develop into root tips. Air layering is one method used. In air layering, a branch is bent downward and a section of it is buried.
The growth tip on the end of the branch is allowed to develop normally, but the buried meristem cells develop into root tips. Once the branch has established roots of its own, it can be cut from the parent plant and become independent. When taking cuttings from plants, remove a growth tip and branch from the parent plant and bury the stem for a similar result.
The main difference between air layering and cuttings is that with air layering the branch continues to receive support from the parent plant, whereas with cuttings the growth tip is separated from the parent plant before the roots are established.
Cuttings tend to need a lot of moisture to survive, at least until they have developed a root system of their own. Cuttings are particularly sensitive to drying out and terminally wilting. They must not be kept too wet or fungal problems such as rots can develop before the plant can become established.
To assist in rooting, sometimes a rooting hormone auxin such as indolebutyric acid or naphthaleneacetic acid is used. Although many cuttings will take directly in soil, it is not uncommon to start cuttings in a deep water culture, aeroponic or hybrid systems. Tomato plants are usually easy to reproduce by cuttings and can be helpful in “proof of concept” attempts at rooting cuttings. Note that taking cuttings of certain patented life forms may be illegal, so only take cuttings from heirloom or other legal issue-free plants.
Some plants naturally use a reverse air layering technique to self-propagate. They send out shallow roots (runners) that can become exposed, triggering root meristem cells to become shoot meristem cells. This will cause new shoots to develop from the root, which may remain connected to the parent plant, or develop independently if the root connection becomes severed.
When a plant becomes sexually mature during flowering, the meristems at budding nodes differentiate into floral meristems. It is the floral meristems that create the flowering and reproductive organs that are used in normal sexual reproduction in plants.
To summarize, inside each seed is a small plant, living and waiting for conditions to initiate sprouting. Germination triggers meristem cell division, used for plant growth. Meristem cells are the stem cells of the plant world. They are important throughout the life of the plant from the tiny, embryonic plant encased in a seed coat, to a mature plant forming flowers and fruits.