Filling, Stiffening, Plumbing
Beyond the epidermis, the plant body is built from three ground tissues and two vascular ones.
Parenchyma is the general-purpose tissue: living cells with thin primary walls, doing photosynthesis, storage and repair. It retains the ability to divide, which is why a cutting can regenerate a whole plant.
Collenchyma has unevenly thickened primary walls and is alive at maturity. It provides flexible support to growing organs, the strings in a celery stalk are collenchyma, and being alive it can extend as the organ grows.
Sclerenchyma has thick lignified secondary walls and is dead at maturity. It provides rigid support, in long fibres or in stubby sclereids: the grittiness of a pear is sclereids.
The two vascular tissues run the plumbing.
Xylem carries water and minerals upward in dead, hollow, lignified cells. Tracheids are narrow and taper, passing water through pits; vessel elements are wider and stack into continuous open tubes. Vessels move far more water, and the reason is dramatic. Flow through a tube follows Poiseuille's law:
Doubling the radius multiplies flow by . Tripling it multiplies flow by 81. A small change in vessel diameter transforms conducting capacity, which is why the evolution of wide vessels was so consequential, and why the trade-off against cavitation risk matters so much.
Phloem carries sugars in both directions, from source to sink, in living sieve-tube elements. These lose their nucleus at maturity and are kept alive by adjacent companion cells: an unusual arrangement in which one cell maintains another's metabolism.
In organography these tissues arrange differently by organ. Roots have a central vascular cylinder that resists pulling; stems have vascular bundles arranged for bending resistance; leaves spread them as veins through photosynthetic tissue.
Common pitfall: assuming a wider xylem vessel is simply better. It moves far more water, but a wide column of water under tension is much more vulnerable to cavitation, an air bubble breaking the column and disabling the vessel permanently. Plants in dry or freezing climates often have narrower, safer vessels on purpose.