the resources to the root to make more roots. Root shortages – the idea here is that if the root is not getting enough shoot derived sugar and gases it will kill part of itself off and send resources to the shoot, to allow more shoot growth. This is an oversimplification, in that it is arguable that some shoot and root cells serve other functions than to acquire nutrients. In these cases, whether they are pruned or not would be "calculated" by the plant using some other criteria. It is also arguable that, for example, mature nutrient-acquiring shoot cells would have to acquire more than enough shoot nutrients to support both it and its share of both shoot and root cells that do not acquire sugar and gases whether they are of a structural, reproductive, immature, or just plain, root nature. The idea that a plant does not impose efficiency demands on immature cells is that most immature cells are part of so-called dormant buds in plants. These are kept small and non-dividing until the plant needs them. They are found in buds, for instance in the base of every lateral stem. Theory of hormonal induction of senescence There is little theory on how plants induce themselves to senesce, although it is reasonably widely accepted that some of it is done hormonally. Plant scientists generally concentrate on ethylene and abscisic acid as culprits in senescence, but neglect gibberellin and brassinosteroid which inhibits root growth if not causing actual root pruning. This is perhaps because roots are below the ground and thus harder to study. Shoot pruning – it is now known that ethylene induces the shedding of leaves much more than abscisic acid. ABA originally received its name because it was discovered to have a role in leaf abscission. Its role is now seen to be minor and only occurring in special cases. Hormonal shoot pruning theory – a new simple theory says that even though ethylene may be responsible for the final act of leaf shedding, it is ABA and strigolactones that induces senescence in leaves due to a run away positive feedback mechanism. What supposedly happens is that ABA and strigolactones are released by mostly mature leaves under water and or mineral shortages. The ABA and strigolactones act in mature leaf cells however, by pushing out minerals, water, sugar, gases and even the growth hormones auxin and cytokinin