On one hand have plants no as efficient transport system as the blood circulation, on the other hand could no hormone that covers all mentioned criteria be isolated, and thirdly have plants no equivalent to the central nervous system of animals for the integration and co-ordination of all physiological activities. Still, plants have regulated growth, plainly determined steps of differentiation, different metabolic rates in cells, and at least partially a communication between cells, too. The cellular exchange of material is ensured by perforations of the cell walls at regular intervals. The search for suitable regulator molecules or effectors was successful. Even if certain classical definitions do not apply is it spoken of plant hormones or phytohormones .
More cautious people do also speak of growth regulators.
In any case is no regulated plant growth possible without them. Plant hormones are without exception small molecules. A number of results indicate that phytohormones enter cells and regulate intracellular processes, though hardly anything about their intracellular distribution or about their transport from one compartment into another is known. It remains open, too, whether they are stored in one or the other compartment, and whether they become biologically active by being set free from such compartments. The second messenger concept seemed not to work in plant cells.