Grodan Stonewool Macroplugs 

Macroplugs are great for plant cloning or starting seedlings. 

The round stonewool (rockwool) plug has a slit from top to bottom that lets you easily insert a plant cutting or a seed.

The fibrous structure of a Macroplug creates an ideal air-to-water ratio that encourages robust root development.

When the roots of your plants start to outgrow the plug, simply place the Macroplug into the pre-drilled Delta Gro-Blocks to enlarge the growing media area.

Macroplugs measure 1-1/2" in diameter.

More info from the Manufacturer (PDF format - Lots of detail.)

Grodan's easy seed and plant cloning suggestions.