I got an email from someone saying that my plans were hard to follow for the cactus farm.
I added two more levels; so I took some new screen captures and I'll use them to try and explain more clearly:
Lay the sand out as shown in the foreground. Place 2 cubes of glass on top of the sand blocks sticking up. Dig the sand out and place a torch in its place. Plant a cactus beside each torch.
Note: in the background you can see a block of dirt. I had that there as scaffolding and removed it when I was done.
The torches aren't necessary but they look good and help keep monsters away. One creeper could really do a lot of damage.
I now have 192 cacti growing in the farm. More than enough for my purposes.
Inside the farm looking up.
I labeled the water channels the same as my plans. What I meant is the number of blocks each channel would need to be dug down, This way the cacti will drop from one channel to another and be transported.
Here the cactus are being transported to Gravehome. The torches are where the channel goes underground.
And this is a top view. Glass cannot cover each story as the cactus needs to be able to fall into the water channels below.
And as always: you can download this world by going to Downloads and Archives. The world is full of surprises I haven't yet posted about.