The genetic make-up of the tall parent can be depicted as $TtWW$ .

Since all the progeny bore violet flowers, it means that the tall plant having violet flowers has $WW$ genotype for violet flower colour.

Since the progeny is both tall and short, the parent plant was not a pure tall plant. Its genotype must be $Tt$.

Therefore, the cross involved in the given question is

$TtWw$ $\times $ $ t tww$

                    $\downarrow $

$TtWw$ $- $ $ t tww$

Therefore, half the progeny is tall, but all of them have violet flowers.