Question

In a dicot stem, secondary growth occurs due to the activity of
a. Apical meristem
b. Lateral meristem
c. Cork
d. Bark

Answer 1

Hint: Secondary growth in dicots increases the diameter of the stem. In woody plants, secondary tissues form the bulk of the plant. Since they grow much taller than other plants, their stems and roots need more support which is provided by secondary tissues. Beside this, it also provides protection, and helps in conduction of water and nutrients to the upper sections of the plant.

Complete answer:
Dicot plants show two types of growth-
• Primary growth: This type of growth produces increase in length and development of lateral appendages.
• Secondary growth: It is the formation of secondary tissues from lateral meristem that increases the diameter of the stem rather than its length.

Secondary tissues are formed by two types of lateral meristems-
• Vascular cambium: It produces secondary vascular tissues
• Cork cambium: It is also called phellogen and it forms periderm. The secondary tissues produced by it constitute the growth in girth or thickness of the plant.
• Phellogen divides in a bipolar way to form secondary tissues, i.e. the cells divide on the inner side as well as the outer side. The secondary tissue produced in the inner side of the cork cambium is parenchymatous or collenchymatous in nature. It is called phelloderm or secondary cortex. Its cells show radial arrangement.
• On the outer side, phellogen produces phellem or cork. It consists of compactly arranged rectangular dead cells that have suberised cell walls. The cork cells contain tannins which makes them appear brown or dark brown in colour.
• The phellem, phelloderm and phellogen together makes the periderm.
• As long as the lateral meristems continue to divide and produce new cells, the girth of the plant keeps increasing.

Hence, the correct answer is option (B).

Note: Some important points to remember-
• Lateral meristem like cambium occur between primary phloem and primary xylem in the vascular bundles of dicotyledonous and gymnosperm stems.
• Secondary growth is not seen in all plants. It is seen in gymnosperms and most dicot plants (woody dicot plants). Only a few monocots show secondary growth and none of the pteridophytes.