plasmodesmata

Plant cells live inside stiff boxes — their cell walls — which would seem to wall each cell off completely from its neighbors. Yet a plant is one connected organism, and its cells must share food and signals. The solution is to leave doorways through the shared walls. Plasmodesmata are exactly those doorways: tiny channels that pierce the cell walls so the living interiors of neighboring plant cells flow into one another.

Each plasmodesma is a narrow tunnel lined by the plasma membrane, threading through a pore in the cell wall so that one cell's membrane is continuous with the next. Running down the middle of many of them is a thin strand of endoplasmic reticulum called the desmotubule, leaving a sleeve of cytoplasm around it through which water, ions, sugars, and signaling molecules can pass. Because all the cells are linked this way, much of a plant becomes one continuous living network — called the symplast — that the cell walls would otherwise divide. Plants can adjust how wide these channels open, and they regulate what is allowed through.

Plasmodesmata are the plant world's answer to the animal gap junction: both are channels that connect the insides of neighboring cells, but plasmodesmata thread through cell walls and can pass larger cargo, even some proteins and RNA. This matters a great deal: nutrients move from cell to cell through them, developmental signals spread across a growing leaf, and — less helpfully for the plant — some viruses hijack plasmodesmata to spread from cell to cell. The honest caveat is that they are not simple open holes; the plant actively controls them, and they can be plugged shut to wall off a damaged or infected region.