From what I have seen, it is from the use of hydroponics or aeroponics, like ForgetClassC said.

With no soils, and only liquids and gases being provided for the plant, more nutrients can easily be absorbed into the roots then into the stem.
kshchrn831 Reviewed by kshchrn831 on . Hollow Stem?? Ok we have pretty much only been growing 1 strain and been happy with that but we finally got to the point where we wanted a lil variety. So on our tent grow we sprouted a "freebie" seed from our last order and have it growing with 4 Medibud plants. Now we have no clue what strain this plant is and we have not dealt with different strains while growing. Last night we took a clone from this unknown plant and when I went to scar the stem before putting into a RR I saw that the stem was hollow. Is Rating: 5