30.4 Leaves

Carrots, turnips, and beets are examples of tap roots that have been modified for food storage.

Many vegetables have changed roots.

Epiphytic roots allow a plant to grow on another plant.

The epiphytic roots of orchids absorb water.

The banyan tree is an Epiphyte in a host tree.

The growing plant strangles the host tree when the aerial roots extend to the ground.
The aboveground roots of the screwpine help support the plant in sandy soils.

Plants use leaves as the main sites for synthesis of food.

Most leaves are usually green.
Some leaves may have different colors due to other plant pigments.

The leaves are adapted to the environment.

Each variation helps plants maximize their chances of survival.
Plants growing in tropical rainforests have larger surface areas than plants growing in deserts or very cold conditions, which are likely to have a smaller surface area to minimize water loss.

Most leaves have a midrib that travels the length of the leaf and branches to each side.

The margin is the edge of the leaf.

A leaf is a very efficient structure.

Monocots and dicots have different patterns of venation.

The veins run in straight lines across the length of the leaf.
The veins of the leaf form a pattern known as reticulate venation.
The Ginkgo biloba has a venation where the veins fork.

The leaf has a netlike venation.

The Ginkgo biloba tree has two different venations.

Depending on the species, the number and placement of a plant's leaves will vary.

There are either alternate, spiral, or opposite leaves.
Plants with only one leaf per tree have leaves that are either alternate or spiral, meaning the leaves alternate on each side of the stem.
In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch.

The leaves can be simple or compound.

Each leaflet is attached to the rachis.
The leaves of poison ivy and the buckeye tree are examples.

It is possible that leaves are simple or compound.

The lamina is always present in simple leaves.
The lamina is separated into leaflets in compound leaves.
There are compound leaves that are palmate or pinnate.
The leaflets branch from the petiole in palmately compound leaves.
In pinnately compound leaves, the leaflets branch from the midrib.
There are double compound leaves on the honey locust.
The upper and lower layers of the leaf are called the epidermis.
The upper side of the adaxial surface and the lower side of the adaxial surface are referred to by biologists.
The skin helps regulate gas exchange.

Two guard cells regulate the opening and closing of the stoma.

With a scanning electron microscope, you can see that several stomata are visible on the surface of the sumac leaf.

The guard cells of a single lyre-leaved sand cress have lips that surround the opening at 5,000x magnification.
The guard cell pair is visible along with the large, sub-stomatal air space in the leaf, in this (c) light micrograph cross-section of an A. lyrata leaf.

In plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration.

The rate of water loss from the leaf surface is reduced by the cuticle.
There may be small hairs on the leaf surface.
The rate of transpiration can be reduced by blocking air flow across the leaf surface and by storing toxic or badtasting compounds.

There is a fuzzy appearance to the leaves of Trichomes.

There are two types of leaf trichomes: branched and multibranched.

The palisade parenchyma and spongy parenchyma are found in the mesophyll of most leaves.

The palisade parenchyma may be present in one, two, or three layers.
The cells below the palisade parenchyma are irregular in shape.
These are the cells of the parenchyma.
The air space between the parenchyma cells allows for exchange of air between the leaf and the outside atmosphere.
Intercellular spaces in the parenchyma help the leaf float.
There are two layers of the mesophyll.
The guard cells are the only cells that contain chloroplasts.

The central mesophyll is sandwiched between the upper and lower layers of the skin.

The upper palisade layer consists of tightly packed columnar cells and the lower palisade layer consists of irregularly shaped cells.
Land plants have a waxy cuticle that protects them from water loss.
The leaf layers are visible in the electron micrograph.
There are small bumps in the palisade parenchyma cells.
Chloroplasts are present in the parenchyma, but are not as obvious.

The leaf has bundles of xylem and phloem.

The xylem transports water and minerals to the leaves.
The phloem takes the products from the leaf to the other parts of the plant.
The xylem and phloem tissues are always in a single bundle.

The xylem and phloem can be seen in the leaf bundle from the lyre-leaved sand cress.

conifer plant species that thrive in cold environments have leaves that are reduced in size and needle-like in appearance.
The leaves have sunken stomata and a smaller surface area that help in reducing water loss.
In hot climates, cacti have leaves that are reduced to cacti, which in combination with their stems help to conserve water.
Many aquatic plants have leaves that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels water.

"The Pale Pitcher Plant" episode of the video is part of the series Plants Are Cool, Too, a Botanical Society of America video.

The structure of the leaves, stems, and roots ensures that a plant can get the sunlight, water, soil, and oxygen resources it needs.

Plants can thrive in less than ideal habitats if one or more of the resources is in short supply.

Light is hard to come by in tropical rainforests, since many trees and plants grow close together and block the sunlight from reaching the forest floor.

The broad leaves of many tropical plant species maximize the capture of sunlight.
Epiphytes are plants that grow on other plants.
Plants can grow high up in the canopy of other trees where there is more sunlight.
Epiphytes live in the leaves and branches of the plant.
The pineapple family includes bromeliads, ferns, and orchids.
Epiphytes have specialized tissues that allow them to efficiently capture and store water.

Spanish moss is found in an oak tree.

Plants have special ways of surviving in poor environments.

The Venus flytrap and the pitcher plant grow in low nitrogen areas.
These plants have leaves that are modified to capture insects.
These plants may have evolved to have a supplementary source of nitrogen thanks to the insect-capturing leaves.

The modified leaves of the Venus flytrap can capture insects.

The trap suddenly closes when an insect touches the hairs.
The opening of the pitcher plant is lined with wax.
The bugs crawl on the lip and fall into a pool of water in the bottom of the pitcher.
The plant absorbs the smaller molecule.

swamp plants have adapted to thrive in wet areas where their roots are submerged underwater.

The soil in these areas is not stable and there is not enough oxygen to reach the roots.

Mangroves and cypress trees have pneumatophores, which are upward-growing roots with pockets of tissue specialized for gas exchange.

The aerenchyma of wild rice provides a path for oxygen to diffuse down to the root tips.