As described in Chapter 6 the three organs of vascular plants (roots, stems and leaves) have the same basic structure: a boundary of dermal tissue enclosing ground tissue that has one to many strands of vascular tissue running through it. The three organs differ in the distribution of vascular tissue: in roots it occurs as a single central strand; in stems, the vascular tissue occurs as multiple bundles imbedded in ground tissue; and in leaves the vascular tissue often occurs as a reticulate network of veins or as parallel strands of vascular tissue. In both cases there is ground tissue filling the space between vascular strands and dermal tissue. This basic anatomy is easily seen in asparagus if one trims the base and looks at the cut end. The dermal tissue is the tougher outside, the vascular bundles are seen as small circles scattered in the outer portion of the stem, and ground tissue makes up the rest.
Roots and shoots show two polarities, a radial polarity, meaning that tissues and cells differ as one moves outward from the center (along a radius), and a proximate/distal polarity, meaning that cells at the tips of organs, where they are produced, differ from cells away from the tip, cells which are older. Leaves have a tip to base polarity and often have a top/bottom polarity.
In this chapter, we describe in more detail the plant anatomy of flowering plants resulting from primary growth (growth derived from root or shoot apical meristems), and consider the developmental changes and consequently the patterns shown with age (distance from the apex).
If a root is sectioned along the long axis (i.e., a longitudinal section) its developmental pattern is readily apparent. Near the tip is the meristem, recognized by the small size of cells and by mitotic activity, often evidenced by the appearance of chromosomes. Moving back from the tip (proximally, towards the plant body) one encounters older and more mature cells, recognizable because they are larger, no longer dividing, and possess features that distinguish different cell types, e.g., secondary cell walls of tracheids and vessel tube members. Because there is no more cell division or expansion after one moves a short distance from the root apex, the diameter of a root showing only primary growth is generally constant along its length, except for the terminal few millimeters. (Roots exhibiting secondary growth do increase in diameter and are discussed in the next chapter). Cells that are produced by the root apical meristem expand the most in a distal/proximal direction (up/down, assuming the root is vertical) and produce cells that are elongate in this direction. There is much less expansion radially, hence roots primarily grow longer, not wider, and this growth occurs near the root tip. Even more significant than the expansion of individual cells is the fact that most cell divisions in the meristematic zone divide cells, so that most of the additional cells that are produced are in the longitudinal (distal/proximal) plane. This is similar to how a unicellular filament divides to extend itself, with cell divisions that are perpendicular to the long axis of the filament. Cell divisions in the root apical meristem adds cells in the distal direction and only to a limited extent do roots add cells radially. Most roots are roughly 20 to 100 cells wide (assuming only primary growth) but roots are often millions or trillions of cells long.
Assuming a cell division that adds to the number of cells in the distal/proximal plane, a second key consideration following cell division is whether the cell that remains meristematic (and does not grow) is distal (towards the tip) or proximal (towards the rest of the plant). In the vast majority of cell divisions of root meristematic cells, the cell that remains meristematic is distal, and the expansion of the other cell pushes the meristematic region into the soil. However, in a small portion of cells it is the proximal cell that remains meristematic and the distal cell matures and becomes part of the protective root cap, located at the extreme distal end of the root. The cells of this root cap are continually sloughed off as the root extends through the soil, and the root cap ensures that meristematic cells themselves are not sloughed off.
The Baltic Blue plant (Epipremnum pinnatum ‘Baltic Blue’) is a relatively new variety of pothos that is prized for its large blue-tinged leaves. As a climbing vine, the anatomy and functions of its roots and stems allow it to grow quickly and efficiently in the right conditions. Understanding the physical makeup of this plant helps provide better care.
Overview of Baltic Blue Plant
Baltic Blue pothos is native to the Solomon Islands and was first cultivated by Costa Farms in 2022 It is considered a fast growing vine that can reach up to 10 feet long in ideal conditions The leaves emerge deep green and gradually develop blue hues as they mature. It has similar needs to other pothos varieties, preferring bright, indirect light and average room temperatures.
Anatomy of Roots
The roots of the Baltic Blue pothos are adventitious, meaning they arise from locations other than the main root system The roots emerge from nodes along the stem at leaf joints.
Some key features
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Fibrous, matted structure – allows for greater surface area for water and nutrient absorption.
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Shallow root system – top 12 inches of soil is where most roots grow.
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Extensive network of small lateral roots branching from the main taproot.
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Covered in fine root hairs to aid in absorption.
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Whitish to light brown in color.
Functions of the Roots
The Baltic Blue pothos root system serves several important functions:
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Anchoring the plant – the dense network anchors the vining stems and leaves.
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Absorbing water & nutrients – fibrous structure and root hairs allow for maximum absorption.
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Storing nutrients – roots act as storage organs for carbohydrates and other compounds.
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Transport – roots contain xylem tissues to transport water and nutrients up to the aerial parts.
Anatomy of Stems
The stems of the Baltic Blue pothos are long, flexible, aerial vines that emerge from the root system.
Key anatomical features:
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Slender vines ranging from green to blue in color.
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Nodes occur at intervals where leaves and adventitious roots emerge.
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Internodes are the segments between nodes.
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Contain xylem and phloem tissues for transport.
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Covered in small waxy hairs to reduce water loss.
Functions of Stems
The flexible stems of the Baltic Blue pothos serve several important functional roles:
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Support – provides physical support to leaves and structure to the plant.
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Transport – xylem and phloem tissues transport water, nutrients, and photosynthates.
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Food storage – starchs and other compounds are stored in stem tissues.
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Growth – new leaves emerge from meristematic stem tissues.
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Climbing – flexible stems allow the plant to climb up supports using aerial rootlets.
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Photosynthesis – though limited, stems carry out photosynthesis.
Caring for the Roots and Stems
To care for the roots and stems of your Baltic Blue pothos:
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Provide a sturdy support for climbing stems to attach to.
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Ensure excellent drainage – wet soils lead to root rot.
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Allow soil to partly dry out between waterings to avoid oversaturation.
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Prune congested growth to stimulate new stems and leaves.
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Mist leaves regularly to increase humidity around aerial roots.
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Apply balanced liquid fertilizer monthly during growing season.
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Repot every 2-3 years as needed in well draining potting mix.
With the proper care, the Baltic Blue pothos will quickly grow long, trailing stems and large vibrant leaves to create a beautiful display in your home. Understanding the anatomy and functions of the roots and stems allows you to provide the conditions this plant needs to truly thrive.
Developmental changes in primary root growth
Proceeding proximally from the root tip one encounters the following regions which transition gradually and overlap:
- zone of cell division, the embryonic region, often less than one millimeter
- zone of cell expansion, generally only a few millimeters in extent, a region where cells are elongating, and to a much lesser extent, getting bigger in diameter; .
- zone of cell maturation, a region where cells develop characteristic features. This zone extends from less than a cm to several cm in length. In the youngest part of this zone root hairs are produced but they soon senesce and are lost from the plant
Significant aspects of cell maturation zone include (in order from the tip as one moves proximally):
- conducting elements of the phloem become functional
- the waterproof compound suberin is deposited as a casparian strip
- conducting elements of the xylem become functional
- root hair appearance and disappearance. Root hairs are extensions off of dermal cells. They are produced after these cells have stopped elongating but are present for only a short time before senescing. Hence, root hairs are only present in a relatively small section of the root.
The significance of the casparian strip
These hydrophobic deposits initially occur as a band that blocks water movement through the wall from the outside to the inside. Eventually the entire endodermal cell wall is coated. The casparian strip forces water and any minerals dissolved in water to enter the cytosol at some point in their journey between the soil and the xylem tissue. Before the casparian strip is deposited, i.e., in the youngest part of the root, water can move from the soil to the center of the root through the ‘apoplast’, a term that describes the collective space of cell walls and any water filled spaces between cells, which typically includes at least 10% of the tissue volume. Because the endodermal cells are tightly bound to each other, once the casparian strip is deposited water is forced to move through the symplast in order to cross the endodermis and get to the interior of the root. The symplast is a term that describes the collective volume of the cytosols of all cells, collective because all cells are interconnected by plasmodesmata, membrane bordered cytoplasmic threads that run between cells.
The casparian strip of the endodermis, once deposited:
- allows the plant to regulate, by virtue of the selective permeability of cell membranes, what minerals do and do not enter the xylem tissue, the conduit to the top of the plant
- allows the plant to, under certain conditions, concentrate solutes in the root xylem because the apoplast solution inside the endodermis (and connected to the xylem tissue) is separated by a two membranes (one providing entry into the symplast, one providing exit from the symplast) from the apoplast solution outside the endodermis. Note that the apoplast outside the endodermis is continuous with the soil.
- decreases the ease with which water can move from the soil to the root xylem.
Plant Anatomy and Structure
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