A novel class of compounds which are useful as plant growth regulators is disclosed, having the general structural formula: ##STR1## In which R is cyano C.sub.1 to C.sub.5 alkyl, alkenyl or alkynyl to which may be attached phenyl, halophenyl, methylphenyl, pyridyl, benzoyl, trimethylacetyl, phenoxy, halophenoxy, halobenzoyl, N-phenylcarbamyl, N-alkylcarbamyl, N-trifluoromethylthiadiazolylcarbamyl, carboxy, carbalkoxy or halo substituents,R.sup.1 is H, acetyl C.sub.1 to C.sub.3 alkyl, alkenyl or alkynyl to which may be attached phenyl, halophenyl, carbethoxy, vinyloxy or phenoxy groups, or R and R.sup.1 together may be C.sub.2 to C.sub.4 alkylene,Ar is phenyl or benzoyl,R.sup.2 is C.sub.1 to C.sub.4 alkyl, alkoxy, alkylene, alkylamino or alkylthio; phenoxy, benzyloxy, carbalkoxy, acetyl, methylenedioxy, trifluoromethyl, nitro, halo or cyano and n may be zero or an integer from 1 to 4, with the provision that at least one position ortho to the point of attachment of a phenyl ring of the Ar structure must be unsubstituted,R.sup.3 is lower alkyl or halo and n' may be zero or an integer from 1 to 4.