1. The incipient plasmolytic concentration of root hair of Brassicachinensis is in proportion to that of the surrounding solution, in which it is growing. 2. Ca protects root hair from its plasmoptysis, but this is faciliated by K. 3. The root hairs, which grow in damp air (aerial root hairs), are able to regulate their osmotic value, to make it proportional to that of the surrounding sugar solution, when they are immersed in it. Root hairs develop sometimes in abnormal forms. 4. Aerial root hairs are injured, when they are directly transferred into water. However, they are protected from such injury, if they are transferred first into a 0.5mol solution and then the concentration is gradually lowered to zero.