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Plant organs

PRIMARY STEM. MONOCOT.

Epidermis
Sclerenchyma
Vascular bundles
Phloem
Xylem
Procambium
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Monocot primary stem
Stem: primary growth. Monocot.
Species: corn (Zea mays).
Technique: paraffin section stained with safranin / Alzian blue.

The image above is from a transversal section of a corn stem showing the vascular bundles scattered in the parenchyma. Individual vascular bundles is a feature telling us that this is a primary stem and their more or less scattered distribution that it is from a monocot plant.

Epidermis is a one cell thick layer made up of slightly cutinized cells. Monocot plant usually have one or two layers of sclerenchyma below the epidermis, instead of collenchyma tissue found in other groups of plants. The stem cortex and medullary region are composed of parenchyma cells, that show chloroplasts if they are near the surface. Vascular bundles are scattered among the parenchyma tissue.

Each vascular bundle is surrounded by sclerenchyma forming the so-called bundle sheet. The primary phloem is almost entirely metaphloem. In some vascular bundles, few crushed cells near the bundle sheet are observed as the remainings of the protophloem. In monocot plants metaphloem it is easy to distinguish sieve tubes from companion cells.

Primary xylem show also some alterations that happened during the early vascular bundle development. There are lysigenous cavities after protoxylem lysis, which is destroyed during the early stem growth. When the primary growth is fully accomplished in the stem, as in the image above, the primary xylem is mostly metaxylem. In corn, metaxylem is composed of two large tracheae or vessels surrounded by sclerenchyma.

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