10.3: Organ Modifications
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Root tissue is derived from the root apical meristem. In some plants, environmental stressors will select for plants whose root tissues perform functions other than water absorption and anchorage. These organ modifications have specific names, depending on what function they serve.
- Storage roots: In most roots, surface area is maximized for water absorption. In a storage root, the volume becomes more important. Cells in the cortex are enlarged and contain leucoplasts.
- Pneumatophores: Gases diffuse 10,000x more slowly in water than in air. In plants that grow in saturated soils, such as mangroves, roots cells need to access oxygen to perform cellular respiration at a rate that they cannot accomplish through water. Pneumatophores are roots that emerge above the surface of the saturated zone to “breathe” (pneumo- means lung) and exchange gases with the environment.
- Adventitious roots: Unlike most roots, adventitious roots emerge from stem tissue. A root apical meristem is derived from tissues in the stem, then root tissues are formed from the RAM, as normal. Adventitious roots can be produced from nodes on horizontal or climbing stems or in response to environmental stressors.
- Prop roots: Prop roots are adventitious roots with the specific function of providing stability to a plant. This might happen in unstable soils, on climbing plants, or in plants that have a shallow root system.
Stem tissue is derived from the shoot apical meristem. In many plants, there is a central stem that lateral stems emerge from. You can distinguish stems from roots by the presence of nodes. You can distinguish lateral stems from leaves by location within the node: stems emerge above the leaf.
Just like with roots, stems can be adapted to a particular function in response to environmental stressors.
- Cladode: A cladode is a stem that has increased surface area to perform photosynthesis. This is usually because the leaves have been modified to some other purpose and are no longer performing photosynthesis. In essence, the stem is imitating a leaf.
- Succulent: In the case of succulence, it is the volume of the stem that increases. Stem tissues develop large, specialized cells called hydrenchyma to store extra water. The plant can access this water for metabolism in periods of drought.
- Tuber: Some stems are modified for storage of starches instead of water. A tuber is an underground stem that can be identified by its nodes (often referred to as “eyes” on a potato tuber). Each node is capable of producing a new shoot.
- Corm: A corm is also modified for storage of starches. A corm is swollen tissue at the base of the shoot with linear nodes travelling across it. From these nodes, papery leaves emerge.
- Rhizome: Some plants produce horizontal stems that are used for asexual reproduction. A rhizome is a horizontal stem that is underground. Roots emerging from the nodes of rhizomes are adventitious.
- Stolon: Similar to a rhizome, a stolon is a horizontal stem used for asexual reproduction. Unlike a rhizome, stolons are formed above the soil surface.
- Thorn: A thorn is a lateral branch that has been modified to protect the plant, usually from herbivory. Thorns have a subtending leaf or leaf scar.
- Stem tendril: A tendril is a lateral branch that has been modified for climbing. A stem tendril will have a subtending leaf or leaf scar.
Leaf tissue is derived from the shoot apical meristem. You can distinguish leaves from stem tissue by location within the node: leaves emerge below the axillary bud, lateral stem, or flower. Under normal conditions, the primary function of a leaf is photosynthesis. However, environmental stressors can select for the following modifications:
- Succulent: Much like in stems, leaves can also be modified for water storage in environments where there is drought. You can distinguish a succulent leaf from a succulent stem because the stem will have nodes (the leaf will not).
- Bulb: Leaves can also be modified to store starch. A bulb, like an onion, is composed of fleshy leaves that surround a short, central stem.
- Spine: A leaf modified to be sharp for protection is called a spine. You can distinguish a spine from a thorn by the location within the node.
- Leaf tendril: Sometimes leaves will be modified for climbing. You can distinguish a leaf tendril from a stem tendril by the location within the node.
- Trap: In environments where nutrients are low, some plants have evolved to capture insects for access to nitrogen, phosphorus, and calcium. There are a few ways plants can achieve this. One is to modify a leaf into some sort of trap.
- Phyllode: Another instance of leaf imitation is when the petiole of the leaf becomes flat and leaf-like to perform photosynthesis. Similar to a cladode, this is usually in response to a modified leaf. The pitcher plant on the right shows a phyllode, leaf tendril, and trap all in one leaf.
- Stipular spines: Another sharp armament against herbivory can be a replacement of a stipule with a spine. These can be distinguished from other sharp modifications, as they emerge in pairs at the base of a leaf.
Other Plant Adaptations
In addition to the major organs, plants can have adaptations to specific tissues, cells, or molecules produced by the plant.
- Prickles: Similar to thorns and spines, prickles are a protective adaptation. In this case, it is a modification of the epidermal (and sometimes cortex) tissue. Prickles can emerge anywhere on the shoot, unlike spines and thorns, which are restricted to nodes.
- Trichomes: Trichomes are epidermal cells that have been modified as hairs. While they can still serve a protective function, they tend to be much smaller and less rigid than prickles.
- Raphides: Some plants have molecular, internal sharp armaments. In plants like Tradescantia, calcium oxalate is crystallized into needle-like structures called raphides.
Chop up the leaf or stem of a Tradescantia and make a squash mount to see raphides within the plant tissues.
- Latex: About 10% of flowering plants have evolved latex production, a sticky substance exuded when plants are damaged. This latex can prevent infection of wounds. Many groups of plants have independently evolved the production of latex gum up the mouthparts of herbivorous insects. In some plants, the latex also includes toxic compounds to aid in defense. For example, in milkweeds (Asclepias) this latex contains neurotoxins.