30.1 The Plant Body

Plant Organ Systems and Tissues

Plants are built from two main organ systems and three tissue types, all working together to carry out photosynthesis, transport nutrients, and provide structural support. Understanding how these parts are organized is the foundation for everything else in plant physiology.

Structure and Functions of Plant Organs

Plants have two interconnected organ systems: the shoot system and the root system.

The shoot system includes all above-ground structures: stems, leaves, and reproductive organs. It handles three major jobs:

• Photosynthesis in the leaves, converting light energy into glucose
• Gas exchange through tiny pores called stomata on leaf surfaces ($CO_2$ in, $O_2$ out)
• Reproduction through flowers, fruits, and seeds in angiosperms, or cones in gymnosperms

The root system includes the primary root and all its lateral branches below ground. Its functions include:

• Anchoring the plant in the soil
• Absorbing water and dissolved minerals from the soil
• Storing carbohydrates and other organic compounds
• Conducting water and minerals upward to the shoot system via xylem tissue

These two systems depend on each other. Roots supply water and minerals that the shoot needs for photosynthesis, while the shoot sends sugars down to the roots for energy and storage.

Meristematic vs. Permanent Tissues

Meristematic tissues are made of undifferentiated cells that actively divide. Think of them as the plant's stem cells. They're found in specific growth zones:

• Apical meristems at shoot and root tips drive primary growth (elongation)
• Lateral meristems like the vascular cambium drive secondary growth (thickening)

Permanent tissues are derived from meristematic cells that have differentiated and (in most cases) stopped dividing. Each type is specialized for a particular function:

• Parenchyma for photosynthesis and storage
• Collenchyma for flexible structural support
• Sclerenchyma for rigid mechanical strength
• Xylem for water transport
• Phloem for sugar transport

The key distinction: meristematic tissues produce new cells, while permanent tissues carry out the plant's day-to-day functions.

Primary Regions of Plant Growth

Plants grow from three types of meristems, each in a different location:

1. Apical meristems sit at the tips of shoots and roots. They're responsible for primary growth, which lengthens stems and roots. Every time a root pushes deeper into soil or a stem grows taller, apical meristems are doing the work.

2. Lateral meristems run parallel to the sides of stems and roots. There are two kinds:

   • Vascular cambium produces secondary xylem (wood) and secondary phloem
   • Cork cambium produces the periderm (bark)
   • Together, they drive secondary growth, which increases the girth of stems and roots. This is why tree trunks get wider over time.

3. Intercalary meristems are found between mature tissues in certain monocots, especially grasses. They allow rapid regrowth at the base of leaves after grazing or mowing, which is why your lawn keeps growing back after you cut it.

Roles of Plant Tissue Types

All plant tissues fall into three categories: dermal, vascular, and ground.

Dermal tissue is the plant's outer covering. During primary growth, this is the epidermis, a single cell layer often coated with a waxy cuticle to reduce water loss. In plants with secondary growth, the epidermis is replaced by the periderm (bark). Dermal tissue protects against water loss, physical damage, and pathogens. It also regulates gas exchange through stomata.

Vascular tissue is the plant's transport system, made of two components:

• Xylem moves water and dissolved minerals upward from roots to shoots
• Phloem moves sugars and other organic molecules from photosynthetic leaves to the rest of the plant

Vascular tissue also contributes structural support, especially xylem, which has thick, rigid cell walls.

Ground tissue fills the space between dermal and vascular tissues. It's made up of parenchyma, collenchyma, and sclerenchyma cells. Depending on location, ground tissue carries out photosynthesis (in leaves), stores starch (in roots and stems), or provides mechanical support.

Simple vs. Complex Plant Tissues

This classification is based on how many cell types make up the tissue.

Simple tissues contain only one cell type:

• Parenchyma: Living, thin-walled cells. The most common and versatile plant cell type. They perform photosynthesis in leaves, store starch in roots, and can even resume dividing if the plant is wounded.
• Collenchyma: Living, elongated cells with unevenly thickened walls. They provide flexible support in growing parts of the plant, like young stems and leaf stalks (petioles).
• Sclerenchyma: Dead at maturity, with thick walls reinforced by lignin. They provide rigid mechanical strength. The two forms are fibers (long, slender) and sclereids (short, irregular, like the gritty bits in a pear).

Complex tissues contain multiple cell types working together:

• Xylem includes tracheids and vessel elements (for water transport), fibers (for support), and parenchyma (for storage)
• Phloem includes sieve-tube elements and companion cells (for sugar transport), fibers (for support), and parenchyma (for storage)

Plant Cell Structure and Function

Plant cells share many features with animal cells but have three distinctive structures:

• Cell wall: A rigid layer outside the plasma membrane, made largely of cellulose. It provides structural support and protection.
• Chloroplasts: Organelles containing chlorophyll where photosynthesis takes place. They capture light energy and convert it to chemical energy.
• Large central vacuole: Fills most of the cell's volume. It maintains turgor pressure (the internal water pressure that keeps the plant upright), stores nutrients and waste products, and helps the cell expand during growth.

Turgor pressure is worth paying attention to. When plant cells are full of water, the central vacuole presses outward against the cell wall, keeping the plant rigid. When water is lost, turgor drops and the plant wilts.

Plant Growth Regulation

Plant hormones are chemical signals produced in small amounts that coordinate growth and development throughout the plant. Three major ones to know:

• Auxins promote cell elongation, especially in shoots. They're also involved in phototropism (bending toward light) and gravitropism (root growth downward).
• Cytokinins stimulate cell division and work with auxins to control the balance between root and shoot growth.
• Gibberellins promote stem elongation and seed germination.

These hormones don't work in isolation. They interact with each other and with environmental signals like light, gravity, and temperature to regulate processes from root growth to fruit ripening.