The leucocytes as free moving cells also come under the influence of such tropisms.

When a small capillary tube having one end sealed is partially filled with the bacteria which produce abscess and placed beneath the skin it quickly becomes filled with leucocytes, these being attracted by the bacteria it contains.

Dead cells exert a similar attraction for the large phagocytes.

Such attraction is called _chemotropism_ and is supposed to be due in the cases mentioned, to the action of chemical substances such as are given off by the bacteria or the dead cells.

The direction of motion is due to stimulation of that part of the body of the leucocyte which is towards the source of the stimulus.