Structure of Stems:
Stems are composed of three tissue types:
- Epidermal tissue-Stems are encased by a transparent epidermis which is usually about one cell thick and which often has trichomes. The trichomes serve various purposes; some plants secrete juices to attract insects, which others have sharp trichomes which often entangle the insects and keep them from feeding while they struggle to free themselves.
- Vascular tissue-As there is no procambium above the youngest leaf primordium, and removing a plant's leaf primordia stops vascular differentiation, substances coming from young leaves control the differentiation of procambium and vascular tissues in stems.
- Xylem and phloem in stems occurs in vascular bundles.
Phloem forms before the xylem and it differentiates on the outside of the bundle.
Xylem forms on the inside of the bundle.
- Vascular bundles are often enclosed by sclerenchyma fibers that differentiate after the internode has finished elongating. But, there is a layer of cells between the xylem and phloem, which remains meristematic. In woody plants and some herbaceous dicots, this layer of cells later becomes part of the vascular cambium, and produces secondary growth. The absence of a vascular cambium in monocots,is important feature.
Vascular bundles in stems are arranged differently in different plants as you can see in the images of monocot and dicot stem cross-sections below:.
- Ground tissue-Between the epidermis and the ring of vascular tissue in dicots is the cortex. Most cells of the cortex are parenchyma. Cortical cells are photosynthetic in plants and often store starch. In dicots, the ground tissue with the parenchyma cells in the center of the stem is specialized for storage and is called pith. Pith cells are often lignified, arranged loosely, and the pith may contain secretory structures such as laticifers. Because monocots have vascular bundles throughout their ground tissue, their stems do not have a discernible pith; the parenchyma cells in monocot stems are referred to simply as ground tissue.
Unlike the root with its pith ringed by vascular bundles, the stem has a continuous cortex punctuated with vascular bundles.
In the close-up of the vascular bundle below, note the lignifed support cells, the large vessels and the single layer of meristematic cells which produces the xylem & phloem
Dicot structure:Unlike the root,the stele is split into a number of vascular bundles,
The pith remains intact and is separated from the cortex by a meristematic layer, the interfasicular ( between bundles) and fasicular vascular cambium ( within the vascular bundles).
The red arrows below indicate the layer where it divides toward the center to form xylem tissue and towards the outside to form phloem tissue.