Where collenchyma works
Collenchyma sits where a plant would bend, and it sits there only while there is nothing harder to do the job.
Where it is found
Just under the surface of a soft stem
In non-woody plants it forms a cylinder a few cells thick, lying immediately inside the epidermis of the stem, running the whole way round.
In the veins of a leaf
In the midribs and veins of the leaves of dicotyledonous plants.
What it does
Support
In dicotyledonous stems, before wood is formed, the collenchyma cells in the cortex carry out the supporting function — so in a seedling the stem is held up by this tissue. In leaves, the collenchyma of the veins supports the leaf blade.
Photosynthesis
The collenchyma of young dicotyledonous stems contains chloroplasts, and those cells carry out photosynthesis.
A cylinder just under the skin is the strongest place to put it
Bend a stem and the outside stretches while the inside squashes; the material furthest from the centre does the most work. Putting the strengthening tissue in a ring near the surface therefore gives the most stiffness for the least material — the same reason a bicycle frame is made of tubes rather than solid rods.