Wednesday, July 16, 2014

Learning | STEM and Stems

Recently, I was searching the library catalog to see what new science books SSJCPL has for young students. One outstanding book is a biography, from a new series of biographies called the STEM Trailblazer Bios. The only one our library system has is the one about Astrophysicist and space advocate Neil deGrasse Tyson.

The term STEM (an acronym for Science, Technology, Engineering and Math,) made me think about plant stems, so I went searching for pictures of plant stems on I hadn't expected to find these beautiful cross-sections of various stems among the photos, but I thought you might like to have a look at them--especially the Creepy Corn Stem!

Zea mays stem (left) and root (right) cross sections - prepared slide from Carolina Biologicals. You can clearly see the difference in the arrangement of the vascular bundles - the stem has an atactostele (scattered bundles) while the root has a eustele similar to what you would find in the stems of dicot plants (but here surrounded by the root tissues pericylce and endodermis).

Corn Sections - Raynox Macro. From BlueRidgeKitties' photostream on Some rights reserved.

Stained Zea mays stem cross section - prepared slide from Carolina Biologicals, observed through the 10x objective of an Olympus IX81 microscope. Is it just my imagination or do those vascular bundles look like creepy brainless skull-like faces?
Creepy Corn Stem, from BlueRidgeKitties' photostream on Some rights reserved.

Mosaic of prepared slide, taken using MicrOcular through Bresser Biolux AL., and processed using Autostitch.
Cotton Stem, from dayglowill's photostream on Some rights reserved.
This image shows details of a vascular bundle from a young stem of a buttercup (Ranunculus sp.), a dicot. The red cells at the bottom of the image are xylem vessels. The smaller vessels in the centre are known as protoxylem, and are those first formed when the stem is elongating. The larger outer cells are called metaxylem. These cells mature later, when the stem has finished elongating. During their development, the cellulose walls of vessels become impregnated with lignin. Lignification supports the vessels and prevents them collapsing under tension. During the process of lignification the cells die and form continuous non-living tubes that transport water and mineral ions from the roots.   Above the xylem is a region stained green, which contains the phloem. Phloem consists of sieve tubes, which transport the products of photosynthesis from the leaves, along with their associated companion cells. Sieve tubes are living cells, but they lack nuclei. Each sieve tube has cytoplasmic connections to a smaller companion cell that provides the energy requirements of the sieve tube.   Between the xylem and phloem are some very small, undifferentiated cells called the cambium.   Cambium within a vascular bundle is called fascicular cambium. The parenchyma cells between vascular bundles can also develop into interfascicular cambium. This results in a complete ring of cambium around the periphery of the stem, which can divide mitotically and differentiate to produce more xylem and phloem, during the process called secondary thickening.

Transverse section of a vascular bundle of a young stem of a buttercup (Ranunculus sp.) Image by John Adds, from Science and Plants for Schools' photostream on Some Rights reserved.

This image shows a transverse section of part of a stem of Sunflower (Helianthus annuus), a dicot. It shows the development of secondary thickening. The first stage in this process involves the fascicular cambium (in the vascular bundles) being connected up by the formation of interfascicular cambium in the parenchyma between the bundles. This forms a complete ring of meristematic tissue, which divides to form rows of secondary xylem and fibres towards the centre of the stem, and rows of secondary phloem sieve tubes and companion cells towards the outside.   In this vascular bundle the red tissue at the top is sclerenchyma. Below this is an area of phloem (stained green), which contains primary phloem to the outside and secondary phloem towards the centre of the bundle. Below the phloem is secondary xylem, which has formed a complete ring of tissue around the stem. The primary xylem, with its large metaxylem vessels and smaller protoxylem, can be seen below the ring of secondary xylem.   The width of the vascular bundle is about 360 µm.   Image by John Bebbington FRPS
Transverse section of part of a stem of Sunflower (Helianthus annuus) showing secondary thickening. From Science and Plants for Schools' photostream on Some rights reserved.

Transverse section of part of a three-year old twig of Lime (Tilia vulgaris) showing the structure of a woody stem, from Science and Plants for Schools' photostream on Some rights reserved.

Rushes are monocots that live in wetlands such as bogs and marshes, and in damp woodlands. This section shows part of a stem of a Rush (Juncus sp.). Plants that are adapted to live in water or wet conditions are known as hydrophytes.   The stem is hollow and contains star-shaped (stellate) cells in a tissue called aerenchyma. The abundant air spaces in aerenchyma allow air to reach the root systems, which are usually submerged in waterlogged soil.   The stem is photosynthetic, with palisade tissue underneath the epidermis, interspersed with groups of fibres (red). Below this cortex are the scattered vascular bundles typical of a monocot.   The diameter of the large vascular bundle is about 170 µm.   Image by John Adds
Transverse section of part of a stem of a Rush (Juncus sp.) showing aerenchyma.  Image by John Adds, from Science and Plants for Schools' photostream on Some rights reserved.

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