Plant Biology: Transport, Hormones and Flowers

A flowering plant has a shoot system above ground, made of stems, leaves and flowers, and a root system below. Its three organs each have a job: roots anchor the plant and absorb water and minerals, stems hold up the leaves and carry materials between roots and shoot, and leaves are the main site of photosynthesis (see OpenStax 30.1).

Every organ is built from three tissue systems (see OpenStax 30.1):

• Dermal tissue covers and protects the plant, like a skin.
• Vascular tissue carries materials: xylem moves water and minerals up from the roots, and phloem moves sugars and other products of photosynthesis.
• Ground tissue fills the rest and does photosynthesis, storage and support.

Plants keep growing all their lives thanks to meristems, regions of cells that keep dividing. Apical meristems at the tips of shoots and roots make the plant longer; lateral meristems make stems and roots thicker (see OpenStax 30.1). Behind a root tip lie three zones: cells divide, then elongate, then mature into specialized cells (see OpenStax 30.3).

Root hairs, thin extensions of root epidermal cells, greatly increase the root's surface area for absorbing water and minerals (see OpenStax 30.3). From there water enters the xylem and rises through the plant.

What lifts it? Not a pump. Water evaporates from inside the leaf and escapes through tiny pores called stomata; this loss of water vapor is transpiration. As water leaves, it pulls on the water behind it. Because water molecules stick together by cohesion, the pull is passed down an unbroken column all the way to the roots. This is the cohesion-tension theory (see OpenStax 30.5). Up to 90 percent of the water a plant takes up may be lost this way (see OpenStax 30.5).

Each stoma sits between two guard cells that open and close it (see OpenStax 30.4). Open stomata let carbon dioxide in for photosynthesis but also let water out, so the plant trades water for food. Guard cells respond to light, the leaf's water supply and carbon dioxide; at night stomata close and transpiration nearly stops (see OpenStax 30.5). A waxy cuticle covers the rest of the leaf and limits water loss (see OpenStax 30.5). Outside the leaf, dry air, wind and warmth all speed up transpiration, while humid air slows it (see USGS).

Phloem carries sugar from a source, where sugar is made or released, such as a leaf in sunlight, to a sink, where it is used or stored, such as roots, young shoots and developing seeds. Storage organs such as tubers and bulbs can be either: a sink while they store sugar and a source when they send it out (see OpenStax 30.5).

The flow is explained by pressure flow (see OpenStax 30.5):

1. At the source, sugar is loaded into the phloem's sieve-tube elements, helped by companion cells.
2. The sugar draws water in from the nearby xylem by osmosis, so pressure inside the phloem rises.
3. The pressure pushes the sap along toward the sink.
4. At the sink, sugar is unloaded and used or stored, and water returns to the xylem.

Unlike xylem, phloem can carry sap up or down, depending on where the sinks are (see OpenStax 30.5).

Plants respond to their surroundings with chemical signals called hormones. Five major groups are usually described: auxins, cytokinins, gibberellins, ethylene and abscisic acid (see OpenStax 30.6). A tropism is growth toward or away from a stimulus.

• Phototropism: shoots grow toward light. Light from one side causes the hormone auxin to collect on the shaded side, where cells elongate more, so the shoot bends toward the light (see OpenStax 30.6).
• Gravitropism: shoots grow up, away from gravity, and roots grow down. Starch-filled bodies called statoliths settle to the bottom of certain cells and tell the plant which way is down; auxin then redistributes to steer growth (see OpenStax 30.6).
• Ethylene is a gas that ripens fruit, turning starch and acids into sugars; it also plays a part in leaves falling. A ripe fruit in a paper bag speeds the ripening of others nearby, and growers use ethylene to control ripening (see OpenStax 30.6).
• Apical dominance: auxin from the shoot tip holds back the side buds, while cytokinins promote bushier growth (see OpenStax 30.6).

Plant life cycles show alternation of generations: a diploid sporophyte, the plant you see, alternates with a tiny haploid gametophyte that makes the gametes. In flowering plants the gametophytes are hidden inside the flower: the pollen grain and the embryo sac inside an ovule (see OpenStax 32.1).

A flower is built in rings. Sepals protect the bud and petals often attract pollinators. Each stamen has a stalk, the filament, topped by an anther where pollen forms. In the center, the carpel has a sticky stigma, a stalk called the style and an ovary holding one or more ovules (see OpenStax 32.1).

Pollination is the transfer of pollen from an anther to a stigma, carried by wind or by animals such as insects, birds and bats (see OpenStax 32.2). A pollen tube then grows down the style to an ovule and delivers two sperm. In double fertilization, one sperm fuses with the egg to form a diploid zygote, which grows into the embryo, and the other fuses with two polar nuclei to form triploid endosperm, a food store for the embryo (see OpenStax 32.2).

The fertilized ovule becomes the seed, and the tissues of the ovary become the fruit, which usually encloses the seeds and helps disperse them (see OpenStax 32.2). A seed may stay dormant until conditions such as moisture, light or temperature are right. When it germinates, the radicle grows into the first root and the seedling lives on stored food until its first leaves can photosynthesize (see OpenStax 32.2).

Most of these ideas came from simple experiments: weighing plants, covering seedling tips and catching what flows out of them.