Secondary growth is due to the action of Lateral Meristems These include the Vascular Cambium in Shoots and Roots of Gymnosperms and Dicots, the Secondary Thickening Meristem in Monocots and the Cork Cambium which occurs in all of the above.
Secondary growth increases the girth of plant organs, This is most obvious in the shoots of large trees which are wide at the base and taper towards the apex. Few plants would attain a height of a few meters without secondary growth in their stems.
Secondary growth is less obvious in roots. However, cursory observations of plants on campus or in nature will quickly verify the presence of secondary growth in the form of Periderm (cork). The thickness of these roots can only be due to the formation of secondary vascular tissues. Roots play an enormous role in supporting shoot systems. Look at the bases of the Giant Sequoia trees below. Their stems are enlarged at the base but without an enormous root system the stems would topple.
We will concentrate on stems now, and will examine roots later.
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Internal secondary growth increases the diameter of stems and roots. The Cork Cambium produces Periderm which covers the surface of expanding stems and roots. Periderm prevents water loss and protects the pant from environmental and pathological insults. |
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