* Testcross: A testcross involves crossing an individual with an unknown genotype with a homozygous recessive individual. In this case, the recessive individual would be short (tt).

* All Tall Offspring: Since all the offspring are tall, this means they all inherited at least one dominant "T" allele.

* Homozygous Dominant: The only way to guarantee all offspring are tall is if the unknown plant is homozygous dominant (TT). If it were heterozygous (Tt), some offspring would have been short.

Here's a simple Punnett square to illustrate:

```

T T

t Tt Tt

t Tt Tt

```

As you can see, all offspring from a TT x tt cross would be Tt (tall).