When Cells Take On a Job

A bacterium does every job itself, and the previous tracks showed both how and why that forces it to stay tiny. A large organism cannot work that way, and not only because of size: a cell that must do everything can be excellent at nothing, since the requirements of contracting, absorbing and signalling pull a cell's design in different directions. The answer is division of labour. Cells specialise, and a group of similar specialised cells doing one job is a tissue — the first level at which specialisation actually buys anything. Tissues combine into organs, which is why a leaf counts as readily as a heart, and organs into systems. This track then takes the tissues themselves, plant first. Plants grow from only a few small regions called meristems: apical ones at the tips of every shoot and root, and a lateral one, the cambium, hidden as a ring inside the stem — which is why a nail hammered into a trunk stays at the same height for twenty years while the trunk thickens around it. What those meristems build includes two transport tissues that share almost nothing: xylem, dead hollow tubes carrying water upward for free, pulled by evaporation from the leaves; and phloem, living cells carrying dissolved sugar in whichever direction it is wanted, loaded actively at a real cost in energy. The track closes on animals and their four tissue families, sorted by what the cells do rather than what the material feels like — which is why bone and blood end up in the same group — and finishes with the two that act rather than hold: muscle, which can only ever shorten and so can only ever pull, and nerve, which moves nothing at all and carries the decision instead.