Inside plants – the stem
Let us face it, the stem must provide multiple functions for the plant. It supports the leaves in suitable positions to allow them to photosynthesise, carries water and possibly nutrients up to the leaves or flowers and sugars down to the roots, it may store useful materials such as carbohydrates, perhaps make food itself (if green) and still keep the plant safe from attack. Quite a task. Add to these the need to expand and grow up, and you see that it is a complex organ.
The basics are simple enough.
- It is covered in an epidermis that may contain protective toxins and covered in a waterproofing waxy coating.
- It has vascular bundles – a package of phloem (organic nutrient transport mainly), a cell division layer (Cambium or meristem) and xylem for mainly water carriage. [In many examples there is a patch of sclenchyma outside the phloem – staining red.]
- The rest of the structure is often filled with a cell type called parenchyma that aid turgidity and have storage jobs.
Sometimes the peripheral zone has photosynthetic cells (chlorenchyma) or cells with thickened cell walls to add strength (collenchyma). The ‘corners’ of square stems are often filled with collenchyma – look at mint or deadnettle stems.
To stop water loss hairs may grow out of the epidermis.
There are two stem designs that are common: Those with vascular bundles that occur as a ring in young stems (dicot plants) and those that are more randomly located (monocots like grasses).
Most photographs you will encounter are of young stems. In older stems the vascular bundles merge to form a continuous ring in dicots, and eventually grow into a woody stem.