408 Prof. H. Marshall Ward. The Relations between 



how the killing is brought about, so far as the final appearances are 

 concerned. We may place the cell, for instance, for a few minutes in 

 hot water (say 55 — 80° C), or expose it to very low temperatures, or 

 to the vapour of chloroform, to acid gases, &c, and in each case the 

 morphological changes are substantially the same.* In the first 

 place, the movements of the protoplasm cease, and the granulation 

 increases, while the whole contracts away from the cell walls into a 

 more or less shrivelled, irregular lump. The cell-sap, previously held 

 in the sap-vacuolesf under pressure in the turgescent living cell, now 

 escapes, and suffuses the whole tissue, evidently because, the structure- 

 of the protoplasm being destroyed, it can no longer be kept in bounds 

 as it was before. It would carry us too far to enter into the discus- 

 sion as to what kind of changes the protoplasmic lining has undergone 

 in its different layers ; it suffices to note the fact that, whereas the 

 living protoplasm was able to regulate the entrance of substances into 

 the cell-sap and their escape from it, it is no longer able to do so 

 when the cell has been killed, and the uncontrolled sap escapes as said* 

 This sap is acid, often strongly so, and contains, among other things, 

 certain bodies, known generally as chromogenes, which, on exposure 

 to the air, undergo oxidation changes which result in the formation of 

 brown colouring matters. We know very little about these chromo- 

 genes beyond the fact of their existence, but the evidence goes to 

 show that they are unstable bodies of various kinds which are present 

 in the cell- sap under such conditions that they are not directly oxi- 

 dised by the passive oxygen dissolved in the sap ; on exposure to the 

 air, however, some substance in the sap acts as an oxygen-carrier, and 

 they undergo the change of colour referred to. J The consequence of 

 this is that the disorganised protoplasm, cell walls, &c, of the tissues 

 thus suffused turn brown, resulting in the well-known colours of dead 

 vegetable tissues. These changes are accelerated by the organic acids, 

 which cause the chlorophyll grains to turn yellow, and then suffer 

 further changes from the oxygen of the air. 



It is, of course, unnecessary to remark that all the rhythmical series 

 of processes connected with the living cell are now put an end to : 

 respiration, metabolism, growth, assimilation have obviously ceased, 

 as have all the other functions of the cell. Moreover, the evaporation 

 of water is no longer controlled by the conditions imposed on it by 



# This, of course, without prejudice as to the sequence of molecular changes 

 which bring about the final result. 



f See Pfeffer, ' Osmotische Unters.' ; de Tries, " Studien ub. d. Wand d. 

 Vacuolen," &c, in addition to the foregoing. 



X See especially Pfeffer, " Beitr. zur Kenntniss d. Oxydationsvorgange in lebenden 

 Zelleu," pp. 447 — 454, where the older literature is collected. The remarkable 

 behaviour of these srbstances in the cell-sap suggests how extremely complex every 

 part (even the presumably simplest) of the organism of the cell must be. 



