Simple and complex
The second question asked of a permanent tissue is not what it does but how many kinds of cell it is built from.
Simple permanent tissue
Made of cells of one kind only. Look at it under a microscope and every cell in the patch is like every other.
Parenchyma, collenchyma and sclerenchyma — the next two skills.
Complex permanent tissue
Made of more than one kind of cell, working together as a unit.
Xylem and phloem — the two skills after that. Each is built from four different kinds of cell.
Why the distinction is worth making
A simple tissue does one job with one kind of cell, so its structure and its function can be read off each other directly. A complex tissue divides the work: in xylem, some cells carry water while others hold the tube open and others again store food. You cannot explain what a complex tissue does by describing any single cell in it.
Three simple, two complex — that is the whole of plant tissue
Once the meristems are set aside, every plant tissue in this chapter is one of five: parenchyma, collenchyma, sclerenchyma, xylem or phloem. Five names, and the classification above is what holds them apart.
Count kinds of cell, not numbers of cells
A patch of parenchyma may contain thousands of cells and is still simple, because they are all the same kind. Xylem may occupy a much smaller area and is complex, because four different kinds of cell sit in it.
One kind or more than one
Simple permanent tissue is built from one kind of cell; complex permanent tissue from more than one. Three simple tissues and two complex ones make up the whole of this chapter’s plant half.