CELLS AND TISSUES OF PLANTS, continued
DERMAL TISSUE - The covering of the plant
Plants that remain HERBACEOUS (i.e., soft and non-woody) throughout their lives maintain epidermis all their lives. However, some plants become WOODY as they age, and in these, the epidermis is replaced by a tougher layer that grows from beneath the epidermis and eventually splits it so that it sloughs off.
The tissues that generate the tougher layer (bark) are LATERAL MERISTEMS, and we will talk about them shortly.
However, let's have a look at what the secondary (i.e., that which replaces the primary epidermal covering) covering looks like.
Here's an overview via microscope sections, of some of the different types of plant cells.
MERISTEMATIC TISSUE: the source of all other tissues.
Before we leap in, here's a nice diagram of THE ANATOMY OF A GENERALIZED PLANT.
APICAL MERISTEM: totipotent tissue located at the tip (apex, hence the name) of every shoot and root.
(A "totipotent" cell can develop into any other kind of cell. Animals have them, too, and they are of great interest to medicine.)The Apical meristem differentiates into the three PRIMARY MERISTEMS:
From inside to outside:
PROTODERM --> EPIDERMIS
PROCAMBIUM --> VASCULAR TISSUE (xylem, phloem)
GROUND MERISTEM --> GROUND TISSUES (parenchyma, collenchyma and sclerenchyma)
(note: meristems located in the "elbows" where leaf meets stem are located in the "axillary buds")
the primary MERISTEMS result in primary growth, that is, increase in height.
A quick look at some of the tissues:
1. Vascular Tissue:
xylem - located on the innermost ring of the stem/root; it carries water and minerals from the roots to the rest of the plant.
phloem - located on the outermost ring (of vascular tissue) in the stem/root; it carries dissolved proteins, amino acids, sugars and other complex compounds in any direction (from a "source" to a "sink" as we will learn later)
Located between the two types of vascular tissue is the "VASCULAR CAMBIUM. This is a SECONDARY MERISTEM which remains meristematic, although the rest of the procambium has matured into xylem or phloem.
The vascular cambium produces secondary growth: it increases the girth of the plant by turning into XYLEM towards the inside of the stem/root and into PHLOEM towards the outside of the stem/root.
As the xylem increases in girth, the phloem and bark eventually get "squeezed" so tightly at the outside of the stem/root that they split and slough off.
* secondary xylem (i.e., that produced by the vascular cambium) is permanent
* secondary phloem (produced by vascular cambium) is not permanent; it sloughs off as the plant grows.
Meristems that increase the height of the plant are called primary meristems.
Meristems that increase the girth of the plant (and are themselves generated by the primary meristems) are called secondary meristems.
Many plants LACK secondary meristems. These have only primary growth and never produce true wood (which is only grown by secondary meristem--vascular cambium). They are said to be HERBACEOUS.
Plants that have woody growth from a vascular cambium are said to be WOODY.
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