The straw comprises a tube (11) having a swelling plug (15) provided with a fibrous support agent and a swelling agent combined with the support agent and configured, at least close to a second end (18) of the swelling plug, after a liquid-based substance has come up against its second end, to be in a state of swelling constrained by said tube such that if a first part (26) of the swelling plug of length comprised between 2 mm and 3 mm starting from its second end has come out of the tube via a second end (14) of the tube, whereas a second part of the swelling plug remains in the tube, the first part of the swelling plug becomes decompacted and expands with the second end of the swelling plug which assumes a diameter (d) at least equal to one and a half times the inner diameter of the tube.
