In reviewing the subject of parasitism from the physiological standpoint, little useful purpose would be served by confining attention to the literature of the last few years. References to the physiological relations of host and parasite are often of a casual nature, made in the course of investigations which had some other object in view, and even if one succeeded in collecting and evaluating these rather miscellaneous observations or interpretations, no satisfactory picture of the problem would emerge. It is proposed, therefore, more especially as this is the first of these reviews which deals with the physiology of parasitism, to make a general survey of the literature, in the light of which the more recent developments will be more readily understood. The earliest substantial contribution to the subject is by De Bary (6) in a classical paper dealing with the mode of invasion of Sclerotinia libertiana, a non-specialized type of parasite. Examination of infected tissue (broad bean, carrot, etc.) showed that two types of action had occurred; one, the solution or partial solution of certain constituents of the cell walls whereby the tissues lost coherence or in other words were rotted; and the other, the killing of the protoplasts themselves. De Bary further demonstrated the presence, in extracts of tissues parasitized by the fungus, of an active principle which produced the effects described. Boiling of these extracts in general destroyed their activity, whence the conclusion was drawn that the effect on the cell wall was due to an enzyme. The nature of the toxic or killing substance was less clear and De Bary, in effect, left that question open. In this connection he considered oxalic acid, which is known to be produced by a number of fungi, but the evidence that this acid was produced in sufficient concentration by Sclerotinia libertiana was unsatisfactory. In studying the earliest phase of attack, i.e., penetration of the intact surface of the host, De Bary reached certain conclusions which 236