Abstract: Based on the proportion that exists between the length of a conductive material and its electrical resistance and using a Wheatstone bridge type circuit, with four resistances to which a given input voltage is applied, and when the bridge is in balance the output voltage is zero, consists of using as such resistances respective extension measurement bands, based on a grid (1) mounted on an electrically insulating support (2), in such a way that preferably two of said grids (1) are orientated in a principal direction of maximum deformation, whereas the other two (1?) are in a direction at right angle to the previous one, in such a way that the dimensional variations of said bands, determined by the increase or reduction in the dimensions of the tree or plant to be controlled, create an output voltage, positive or negative, which proportionally corresponds with said measurement increase or decrease.