<< Chapter < Page Chapter >> Page >
Photo (a) shows a plant with many fuzzy white hairs growing from its surface. Scanning electron micrograph (b) shows branched tree-like hairs emerging from the surface of a leaf. The trunk of each hair is about 250 microns tall. Branches are somewhat shorter. Scanning electron micrograph (c) shows many multi-pronged hairs about 100 microns long that look like sea anemones scattered across a leaf surface.
Trichomes give leaves a fuzzy appearance as in this (a) sundew ( Drosera sp.). Leaf trichomes include (b) branched trichomes on the leaf of Arabidopsis lyrata and (c) multibranched trichomes on a mature Quercus marilandica leaf. (credit a: John Freeland; credit b, c: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or “middle leaf.” The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma ( [link] ). The palisade parenchyma (also called the palisade mesophyll) has column-shaped, tightly packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.

 Part A is a leaf cross section illustration. A flat layer of rectangular cells make up the upper and lower epidermis. A cuticle layer protects the outside of both epidermal layers. A stomatal pore in the lower epidermis allows carbon dioxide to enter and oxygen to leave. Oval guard cells surround the pore. Sandwiched between the upper and lower epidermis is the mesophyll. The upper part of the mesophyll is comprised of columnar cells called palisade parenchyma. The lower part of the mesophyll is made up of loosely packed spongy parenchyma. Part B is a scanning electron micrograph of a leaf in which all the layers described above are visible. Palisade cells are about 50 microns tall and 10 microns wide and are covered with tiny bumps, which are the chloroplasts. Spongy cells smaller and irregularly shaped. Several large bumps about 20 microns across project from the lower surface of the leaf.
In the (a) leaf drawing, the central mesophyll is sandwiched between an upper and lower epidermis. The mesophyll has two layers: an upper palisade layer comprised of tightly packed, columnar cells, and a lower spongy layer, comprised of loosely packed, irregularly shaped cells. Stomata on the leaf underside allow gas exchange. A waxy cuticle covers all aerial surfaces of land plants to minimize water loss. These leaf layers are clearly visible in the (b) scanning electron micrograph. The numerous small bumps in the palisade parenchyma cells are chloroplasts. Chloroplasts are also present in the spongy parenchyma, but are not as obvious. The bumps protruding from the lower surface of the leave are glandular trichomes, which differ in structure from the stalked trichomes in [link] . (credit b: modification of work by Robert R. Wise)

Like the stem, the leaf contains vascular bundles composed of xylem and phloem ( [link] ). The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.

 The scanning electron micrograph shows an oval vascular bundle. Small phloem cells make up the bottom of the bundle, and larger xylem cells make up the top. The bundle is surrounded by a ring of larger cells.
This scanning electron micrograph shows xylem and phloem in the leaf vascular bundle from the lyre-leaved sand cress ( Arabidopsis lyrata) . (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Leaf adaptations

Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area: two attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels water.

Questions & Answers

what is biology
Dada Reply
The scientific study of life.
the virus that causes mumps in humans is composed of a protein outer Shell containing a core of DNA
Daniel Reply
Basic science and applied science question about cancer
Joyce Reply
what are the importance of ATPs
Olatunji Reply
How can biology be studied from a microscopic approach to a global approach
Joyce Reply
The large central opening in the poriferan body is called
Chynna Reply
You go for a long walk on a hot day. Give an example of a way in which homeostasis keeps your body healthy.
Joyce Reply
You sweat.
sweating is your bodies way of keeping you from overheating.
Thank you
what is biology
Neya Reply
biology is the study of life
Biology is the study of Life
is the branch of science which deals with the of living things.
what is metabolism
Sangam Reply
hi palz, what's the ?
Gbolajobi Reply
hi plz, what's the.
pls come a again
Describe the steps and results of reintroducing wolves to Yellowstone National Park.
Natalia Reply
tag and release wolves into Yellowstone. wolves eventually reproduce and the pack grows. as wolves hunt they cull the sick and weak prey. the carcass that is left provides food for other species (scavengers and insect.. etc). this heals the circle of life and contributes to the biodiversity...
before you know it species that are critical to the eco system return. having apex predators is crucial to an ecosystem... it helps run the deer and elk , etc around.
example: there was a species of shrub/ plant that grows along river banks that moose love to eat.. the moose have no predator so they decimate that food source which also helps prevent erosion. when the wolves were reintroduced this changed. oddly enough this plant species started to repopulate in
the areas where wolf feces sat and decayed
which of the following statements about the parts of an egg are false?
Israel Reply
Monotremes include...?
medicinal plants including microbs
vijay Reply
,medicinal plants including microbes
what is biology
Siyanbola Reply
study of living organisms...
study of plants and animals
what is abiotic and biotic factors?
Hira Reply

Get the best Biology course in your pocket!

Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?