Question

how do trees grow?

122 4. How do trees grow? (Focus on the relationship between primary and secondary growth, and between the production of xylem (wood) and phloem (bark, see below).

Answer 1

Primary and secondary growth contribute to the complete growth of tree. Primary growth results in increase in length (taller/longer) of trees and secondary growth results in increase in thickness or girth of the trees.

1. Primary growth

a. Roots

Growth of root begins with seed germination. The radicle of the embryo which emerge from the seeds forms the root system. The tip of the root is protected by the root cap.

The root tip can be divided into three zones:

(i) The zone of cellular division, which contains the apical meristem, (ii) The zone of cellular elongation is the location where the newly formed cells are growing, (iii) The zone of cellular maturation where newly elongated cells complete their differentiation.

b. Shoots

The process of primary growth is controlled by the primary meristems. they allow growth in length or height, which is known as primary growth.It is differentiated into three types i.e protoderm produces new epidermis, ground meristem produces ground tissue, and procambium produces new xylem and phloem.

2. Secondary growth

The process of secondary growth is controlled by the lateral meristems or Secondary meristems.

The two types of secondary meristem (cambium) are

a. Vascular cambium produces secondary xylem:tracheids and vessel elements (toward the center of the stem or root) and secondary phloem:sieve elements and companion cells (toward the outside of the stem or root), adding growth to the diameter of the plant. The cells of the secondary xylem contain lignin, the primary component of wood, which provides hardiness and strength. The xylem together with the pith form the wood of a woody stem.This process produces wood, and builds the sturdy trunks of trees.

b. Cork cambium (outermost lateral meristem) lies between the epidermis and the phloem, and replaces the epidermis of roots and stems with bark, one layer of which is cork.Cork cells contain a waxy substance that can repel water. The phloem and cork cells together constitute the bark, which provide protection to the plant against physical damage and reduce water loss. Phelloderm is another layer of cells produced by the cork cambium which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm.