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Leaves consists of epidermal, ground, and vascular tissues.1. Epidermis-it is compact, transparent, and usually not photosynthetic.
It also contains many stomata. In horizontally oriented leaves, there are usually more stomata on the protected lower side that the exposed upper side. Vertical leaves usually have similar numbers of stomata on both sides. Although stomata occupy 1% of the leaf surface, they lose huge amounts of water in to the atmosphere.
As we saw in lab, the epidemis may also produce large number of trichomes, either simple or glandular to help establish a boundary layer, to reflect light in open areas and to deter herbivores.
2. Vascular-Xylem and phloem in leaves form in strands called veins.
Xylem forms on the upper side of a vein, and phloem forms on the lower side.
Veins are supported by fibers and are usually surrounded by a layer of parenchyma cells called the bundle sheath, which extends to the epidermis of the leaf. These help support the veins and may conduct water to the epidermal cells.
Most dicots and some nonflowering plants have netted venation, meaning they have one or a few prominent midveins, from which smaller minor veins branch into a meshed network.
The leaves of monocots have parallel venation, meaning several prominent and parallel veins interconnect with smaller, inconspicuous veins. Several factors influence the formation of veins, including the presence of other veins.
3. Ground-The ground tissue of leaves is called mesophyll. It contains many types of cells, including
- sclerenchyma,
- storage parenchyma, and
- chlorenchyma.
In general, the arrangement of chlorenchyma is determined genetically, and is influenced by whether the leaf is oriented horizontally or vertically when it forms.
It is usually divided into the palisade layer, with columnar cells, stacked upon each dependent on light intensity, andthe spongy parenchyma, which contains large air spaces for gas exchange ( as well as water)....
II. Internal leaf arrangement:Dicot: has the typical pattern described above
Sunleaf: Note: in the leaf below, the multiple number of palisade layers clue us into the fact that this leaf is exposed to the sun more intensely.
Monocots: as leaves are vertically held, the concept of top and bottom disappears; thus stacking columnar cells above and shaded loose cells below no longer occurs. Here we see both sides exposed to the light, thus no differentiation and stomata on both sides.
We also see an example of Kranz anatomy of the bundle sheath, with a ring of photosynthetic cells surrounding the vascular bundle. Most of the photosynthesis occurs here and not in the general mesophyllic tissue. Later we'll discuss the importance of this anatomy in terms of prevention of photorespiration.
All leaves are modified to some extent...but some especially so for their unique environments..The following example is an 'aquatic' plant whose leaves float on the surface. What modifications can you note?The right image is of a sclerid that appears to support the loose spongy parenchyma cells.
