Assumptions:

* Simple Dominance: We're assuming that long needles (L) are dominant over short needles (l). This means a cactus with at least one "L" allele will have long needles.

* Heterozygous Parent: The problem doesn't specify the genotype of the parent cacti, so we'll assume both parents are heterozygous (Ll), meaning they carry one dominant (L) and one recessive (l) allele.

Punnett Square:

To figure out the possible offspring, we use a Punnett Square:

```

L l

L LL Ll

l Ll ll

```

Phenotype Ratio:

* LL and Ll: Both genotypes result in long needles (L dominant)

* ll: Results in short needles

Looking at the Punnett Square, we have:

* 3 Long needles: LL, Ll, Ll

* 1 Short needles: ll

Therefore, the phenotype ratio is 3:1 (Long Needles: Short Needles)