Apical Meristem

Apical Meristem Definition

The apical meristem is the growth region in plants found within the root tips and the tips of the new shoots and leaves. Apical meristem is one of three types of meristem, or tissue which can differentiate into different cell types. Meristem is the tissue in which growth occurs in plants. Apical is a description of growth occurring at the tips of the plant, both top and bottom. Intercalary meristem is found between branches, while lateral meristem grow in girth, such as in woody plants.

Apical meristem is crucial in extending both access to nutrients and water via the roots and access to light energy via the leaves. Plants must expand in both of these directions in order to be successful. Some plants show apical dominance, in which only one main shoot apical meristem is the most prominent. In plants like this, there is a single main trunk which reaches to great heights. If the apical meristem is cut off, the branches below will start to assume the role of primary apical meristem, which will lead to a bushier plant. Horticulturalists use this phenomenon to increase the bushiness and yield of certain agricultural crops and ornamental plants.

Apical Meristem Function

The apical meristem, found just below the surface of the branches and roots furthest from the center of the plant, is continually dividing. Some cells divide into more meristematic cells, while other cells divide and differentiate into structural or vascular cells. There are two apical meristem locations in most plants. The shoot apical meristem is found in the tips of plants. This apical meristem is responsible for creating cells and growth to drive the plant into the light and air, where it can photosynthesize and exchange built up gases.

The root apical meristem is found at the tips of roots. Sensing the conditions of the soil around the root, signals are created within the apical meristem which direct the plant towards water and desired nutrients. It is for this reason that roots often invade pipes for water and drainage, which carry many of the nutrients they need. The apical meristem, protected by the root cap continues to produce cells even as the root cap is scraped away as it pushes through the dirt. The apical meristem must produce enough cells to not only extend into the soil, but also to replace the cells lost to abrasion.

Apical Meristem Structure

The apical meristem is located just below the root cap in the roots, as seen in the image below. The actual apical meristem is a cluster of densely packed and undifferentiated cells. From these cells will come all of the various cell structure the plant uses. An undifferentiated apical meristem cell will divide again and again, slowly becoming a specialized cell.
Apical Meristem

In the root apical meristem, the cells are produced in two directions. In the shoot apical meristem, cells are only created in one direction. The shoot apical meristem may exist at the tips of plants, as in many dicots, or may start slightly below the soil and generate leaves which grow upward, like most monocots. However, in both groups the shoot apical meristem is the growth center of all above ground growth.

Interestingly, the shoot apical meristem in most plants is capable of producing an entire plant, whereas the root apical meristem cannot. Scientists have used the ability of the shoot apical meristem to clone many species of plant. By simply cutting off the apical meristem and transferring it to an appropriate growth medium, the apical meristem will develop roots and differentiate into a whole new plant. As an added benefit, more apical meristems form on the plant, and can be harvested for more clones. In this way, a desirable plant can be replicated almost indefinitely.

Regulation in the Apical Meristem

Diversification of cells in the apical meristem is a complex process controlled by a number of genes. In effect, these genes determine the shape and structure of a plant. As the apical meristem grows, it branches of smaller meristem locations, which will develop into branches of the stems and roots. The timing and number of these events are controlled by a series of genes within plants. The various expressions of these genes leads to different forms, some of which are more successful than others. The interaction between these genes and the growth of the apical meristem has led to the millions of different species of plants which exist today.

The variety of forms in plants is attributable almost solely to the differences in how their apical meristem functions. Some plants, like bushes, branch continuously and equally, while plants like pine trees have a single main branch. The root apical meristem is likewise responsible for root development. Roots can be deep, and focused on a single branch, such as tap-root, common to many weeds. Corn and bamboo, on the other hand, has much more dispersed and fibrous root system, which depends on lots of branching and lateral roots.

Quiz

1. What is the difference between an apical meristem and an intercalary meristem?
A. No difference
B. The apical meristem is at the tip
C. Intercalary meristems can be apical

Answer to Question #1
B is correct. The term apical simply means at the tip. A meristem is simply a portion of the organism with stem cells. Intercalary describes the space between apical meristems, in which smaller branches form.

2. How can the apical meristem be manipulated to increase the harvest of a crop?
A. They can be cut to create a bushy plant
B. More meristems means more fruit
C. They can’t be manipulated

Answer to Question #2
B is correct. While it might also create a bushy plant, most fruits and vegetables are the product of a fertilized flower. Flowers typically form at a meristem. Therefore, if clipping the apical meristem means more meristems, more flowers can be created.

3. How is the apical meristem similar to stem cells in a human fetus?
A. Both have the ability to differentiate
B. They are completely different
C. They divide in the same way

Answer to Question #3
A is correct. Both sets of cells are totipotent, in that they can differentiate into an entire organism. While the apical meristem may stay totipotent, the stem cells in humans typically reduce the stem cells to multipotent, able to only transform into a handful of related cell types. This is one reason it is much harder to clone a human.

References

  • Hartwell, L. H., Hood, L., Goldberg, M. L., Reynolds, A. E., & Silver, L. M. (2011). Genetics: From Genes to Genomes. Boston: McGraw Hill.
  • McMahon, M. J., Kofranek, A. M., & Rubatzky, V. E. (2011). Plant Science: Growth, Development, and Utilization of Cultivated Plants (5th ed.). Boston: Prentince Hall.
  • Reece, J. B., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., & Jackson, R. B. (2014). Campbell Biology, Tenth Edition (Vol. 1). Boston: Pearson Learning Solutions.
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