Plants can use sugars at the root level; dissolved in water, it's a liquid form of carbohydrates. In plant tissue culture, plants can get their carbon either in the form of gas such as carbon dioxide (a problem because the gas flow has to be sterile, so filters must be used), or in the gelled nutrient substrate that is used.
Usually it's sucrose (table sugar), otherwise it's glucose (dextrose, blood sugar, whatever you want to call it).
Molasses has the advantage of adding iron, probably in a form particularly "friendly" to plants because it is expressed from plant material in the first place.