Question

The collective term used for phelloderm (secondary cortex), cork cambium (phellogen) and cork (phellem) is (a) pericyde (b) periderm (c) protoderm (d) procambium.

Short answer

The collective term for phelloderm, cork cambium, and cork is (b) periderm.

Step-by-step solution

Identifying the Correct Term

To solve this exercise, you need to recall the botanical terms related to plant secondary growth. The terms given in the options are specific to different plant tissues involved in growth and development. Phelloderm, cork cambium, and cork are all part of a plant's secondary protective tissues.

Understanding Tissue Functions

Phelloderm, cork cambium, and cork have specific functions in plants: Phelloderm (secondary cortex) is a layer of parenchyma cells produced by the cork cambium; Cork cambium (phellogen) is a layer of meristematic tissue that produces the cork; Cork (phellem) is the protective outer layer produced by the cork cambium. All these layers together form a protective covering for the plant.

Selecting the Correct Option

The collective term for phelloderm, cork cambium, and cork is 'periderm'. This term refers to the multi-layered secondary protective covering that replaces the epidermis in stems and roots of woody plants during secondary growth.

Key concepts

Secondary Growth in Plants

Plants have two types of growth: primary and secondary. Primary growth is all about elongation, helping the plant to grow taller and the roots to extend deeper into the soil. But as plants mature, particularly in woody plants, they start outgrowing their own skin. This is where secondary growth comes into play; it's more about girth than length. Secondary growth is facilitated by two types of lateral meristems: vascular cambium and cork cambium.

The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem, while the cork cambium (also known as phellogen) produces the periderm, an outer protective layer that replaces the epidermis in older stems and roots. Think of secondary growth like a tree getting buffer, developing a thick, protective bark and sturdier vascular system to support its increasing size and height.

Cork Cambium (Phellogen)

When you hug a tree, you're actually getting up close and personal with the handiwork of the cork cambium, or phellogen. It's a meristematic tissue—which means it's a region of active cell division—situated just beneath the bark of the tree. The cork cambium's superpower is its ability to produce cork cells to the outside, which eventually turn into the rugged, protective bark, and phelloderm to the inside, a much softer layer of cells. This process allows the stem to expand and provides a durable armor against physical damage and prevents water loss.

These cells are packed with suberin, a waxy substance that makes them water-resistant and tough. But the cork cambium isn't a one-trick pony — it's a key player in the tree's ability to grow wider and stand strong for years, or even centuries.

Phelloderm (Secondary Cortex)

Let's delve a bit into the phelloderm, often referred to as the secondary cortex. This layer is the softer side of the periderm, produced by the cork cambium but to the inside. It's largely made up of parenchyma cells, which are like the utility players in the world of plant cells. They store food and help with the transport of nutrients and water. Phelloderm adds some extra padding to the plant's structure, giving it more resilience and, in a way, a tad more comfort.

Though it doesn't get the same fame as its tougher counterpart, the cork, phelloderm is crucial. It supports the plant's secondary growth from within, complimenting the protection provided by the outer layers.

Cork (Phellem)

Last but not least, there's the cork, or phellem, the outermost layer of the tree's skin. This is the part you see and touch when you look at tree bark, and it's remarkable stuff. Cork cells are dead at maturity, filled with air or substances like the waxy suberin, which gives cork its characteristic imperviousness to water and gases.

As cork cells accumulate from the cork cambium's activity, they form a thick bark that defends the plant against extreme temperatures, invading pathogens, and the harsh realities of the environment. It ensures the plant doesn't lose too much water, making it an essential feature for survival, especially in fire-prone or arid areas. Cork, which we harvest for various products, highlights the incredible adaptations plants have made to protect themselves through thousands of years of evolution.