Problem 12 - Biology

Question

The outer tissue that covers plants with primary growth is _____________ whereas ____________covers plants with secondary growth. (a) cuticle; cork parenchyma (b) periderm; phloem (c) epidermis; periderm (d) epidermis; collenchyma (e) cellulose; lignin

Step-by-Step Solution

Answer

The answer is (c) epidermis; periderm.

Step 1: Analyze the Question

The question asks us to identify the outer layer that covers plants at different stages of growth. Specifically, it focuses on primary and secondary growth.

Step 2: Understand Primary and Secondary Growth

Primary growth refers to the increase in length of the plant, while secondary growth refers to the increase in thickness or girth of the plant. These stages involve different types of tissues.

Step 3: Identify Tissues for Primary Growth

In primary growth, the outer tissue is the epidermis. The epidermis serves as a protective layer for new growth areas of the plant.

Step 4: Identify Tissues for Secondary Growth

During secondary growth, the epidermis is replaced by a more protective layer known as the periderm. The periderm includes structures like the cork (phellem).

Step 5: Match Answer Choices

Now that we know the terms for each growth stage, let's match them with the options provided: - For primary growth, the outer layer is the **epidermis**. - For secondary growth, it is the **periderm**. Thus, the correct choice is option (c) epidermis; periderm.

Key Concepts

Primary Growth

Primary growth is a key phase in a plant's life cycle. During this stage, plants focus on growing taller or longer. Think of it as the process that helps a plant reach for sunlight and spread its roots deeper into the soil. Primary growth occurs at the tips of roots and shoots. These regions are known as apical meristems.

Apical meristems are like miniature factories that keep the plant growing in length. They constantly produce new cells which expand and push the plant forward. As part of this growth, a protective layer called the epidermis forms on the outside.

• Increase in length
• Occurs at apical meristems
• Forms the epidermis

Secondary Growth

Secondary growth, unlike primary growth, focuses on a plant growing thicker or wider. This is especially important for woody plants like trees and shrubs. The tissues responsible for secondary growth are different from those in primary growth.

Secondary growth happens in a part of the plant called the lateral meristem. The girth increases as new layers of tissue are added. One famous tissue produced during this stage is wood, formed by the vascular cambium.

• Increase in thickness or girth
• Occurs in lateral meristems
• Forms the periderm

Epidermis

The epidermis is the skin of the plant. It covers leaves, young stems, and roots during the primary growth stage. This thin layer acts as a shield, protecting the plant from environmental dangers like pests and harsh weather.

In addition to protection, the epidermis can help manage water loss. In some plants, the epidermis features cuticles—waxy layers that reduce evaporation. It's also involved in gas exchange, where tiny openings called stomata allow the plant to breathe. The epidermis, though a superficial layer, plays vital roles:

• Protection against external threats
• Reduces water loss with cuticles
• Involved in gas exchange through stomata

Periderm

Periderm replaces the epidermis when secondary growth begins in woody plants. It acts as a tougher, more robust skin for the plant. The main component of the periderm is cork. Sometimes referred to as phellem, cork provides excellent protection.

This thick, waterproof layer reduces water loss even further than the epidermis can. Cork is crucial for plant survival in challenging conditions, like bacteria and fungi attacks or physical damages:

• Replaces the epidermis
• Thick and waterproof
• Protection from physical and microbial threats