426 



KNOWLEDGE & SCIENTIFIC NEWS. 



[May, 1906. 



Irritability. 



Reaction of Protoplasm to 

 Stimuli. 



Bv Harold A. Haic, M.B. 



Protoplasm will react to external stimuli and within 

 limits can adapt itself to stimuli of an unusual and 

 powerful nature. But if these latter be too long sus- 

 tained a complete or partial cessation of function will 

 result. 



Let us take the case of a turgescent root-hair sur- 

 rounded with a dilute aqueous solution of various salts. 

 Some of these salts will be selected to the exclusion of 

 others, the protoplasm lining the cell-wall, or, rather, 

 the laver known as the ectoplasm, showing what is 

 known as a selective capacity for certain salts. Now 

 this process shows us that tlie ectoplasm by reason of 

 its reaction to the stimuli of .some salts and not those 

 of others acts as a kind of .sieve, and keeps up the 

 requisite osmotic equilibrium. A non-living membrane 

 shows no such predilection for some salts over others; 

 in fact, there must exist in our layer of protoplasm the 

 capacitv of reacting specifically to some kinds of chemi- 

 cal stimuli over and above others. 



Take again the case of naked masses of protoplasm 

 (Plasmodia) when subjected to the influence of some 

 irritant : retraction will in some cases occur, in others 

 attraction, and here again we find the same kind of 

 specific irritability. These phenomena are not to be 

 explained on purely physical or chemical lines, but are 

 of a complex nature in which both physical and chemi- 

 cal changes occur. Later on we shall see that rapid 

 changes in metabolism have probably a large share in 

 the production of the various external manifestations 

 such as alterations in shape of the protoplasm, move- 

 ments, and so on; but some of the intrinsic molecular 

 changes which take place when a stimulus is applied to 

 protoplasm are so hidden that as yet no satisfactory 

 theorv has been advanced to explain all the changes 

 which take place between a stimulus and its effects. 



In the animal cell we find conditions are somewhat 

 different; in the first place we cannot make any distinc- 

 tion between an outer transparent and an inner granular 

 laver of protoplasm, such as is possible in plant cells, 

 and, moreover, no true cell-waJl exists. Under the 

 influence of certain irritants, as, for instance, the 

 mineral acids, we find that living cells from a given 

 part of the body, such as the skin, will, some of them, 

 become completely disorganised, and adjacent cells 

 show signs of increased rapidity of division, a fact 

 pointing to a reparative process, and due to the stimuli 

 tr.insmitted by neighbouring injured cells. Instances 

 of this reparative process occur at times in plants, as 

 in the case of the production of a cushion of callus 

 round the edges of .-i cut made through the bark of a 

 tree so as to mclud;' the cambium and xylem; and in 

 the overgrowth whic-h occurs in certain cases where a 

 parasite attacks a plant. 



In those cases where animal cells are exposed to the 

 action of the bacterial toxins, a very complex result 

 often ensues; in some cases, as is well known, the cell 

 will resist the poison, and produce substances known 

 as antitoxins, which tend to arrest or neutralise the 

 action of the bacterial toxin. In other cases toxic 



bodies are formed outside the bacterial cells from sub- 

 stances either in the animal cell or from the media 

 surrounding them (extracellular toxins), and these, by 

 their irritant action, either kill the cells or stimulate 

 them to form antitoxins. 



When :i cell becomes incapable of reacting to the 

 irritant, it undergoes a process of degeneration (fatty 

 degeneration), which ends in complete disorganisation 

 and breaking up of the protoplasm into droplets of fatty 

 material. Often certain cells take on a reparative or 

 protective function, and form a laver round colonies of 

 bacteria, enclosing them and finally indirectly affecting 

 their destruction (as in an abscess). 



We see, then, that in both the plant and animal cell 

 a definite reaction takes place whenever the cell is ex- 

 fvosed to an irritant; in the plant cell there is in most 

 cases cither immediate cessation of function, where the 

 irritant is powerful enough, or the occurrence of a 

 physical or chemico-physical process, which tends to 

 remove the protoplasm away from the source of irrita- 

 tion when harmful, and place it in a kind of defensive 

 equilibrium. In the animal cell, on the other hand, the 

 first indication of reaction is one of increased cell-divi- 

 sion (where vascular tissues are concerned there is also 

 an emigration of certain cells, leucocytes, which act 

 partly as phagocytes and aim at destroying the irritant), 

 followed in some cases bv the formation of a layer of 

 cells, which enclose the irritant and proceed with re- 

 pair. Now, it may be asked, why should protoplasm 

 react in this manner to stimuli? The retraction of 

 Plasmodia awav from an irritating substance, and the 

 increa.sed rate of cell-division in the animal cell when 

 stimulated cannot he due to purely physical or chemical 

 causes acting separately. When we say that " irri- 

 tability " in a plant or animal cell is that property which 

 enables it to react to a stimulus of whatever nature, we 

 merely state the bare fact that by reason of it certain 

 changes occur which somehow alter the previous rela- 

 tions of the protoplasm, but we offer no explanation as 

 to why such changes should occur. .\s a matter of 

 fact, it is probable that intrinsic changes do take place 

 in the ultimate physical constitution of the protoplasm, 

 whereby a rearrangement of molecules takes place with- 

 out altering the general chemical constitution. The 

 change, in fact, must be very similar to what takes 

 place when a nerve transmits a stimulus from its peri- 

 pheral to its central end or vice versa ; in this latter 

 process it is known that electrical changes take place, a 

 fact which points to the occurrence of rapidly alternating 

 anabolic and katabolic changes, these manifesting them- 

 selves in a current which travels in a certain definite 

 direction. 



Recentiv it has also been shown that anv li\ing pro- 

 toplasm will manifest electrical changes when irritated, 

 pointing to the same anabolic and katabolic changes, 

 and the quiescent protoplasm of seeds which have lain 

 dormant for years will, when an inciting current is 

 passed through them, show what has been called a 

 "blaze-reaction," that is, an electrical respon.se-cur- 

 rent, which indicates that the protoplasm is irritable 

 and has reacted. (See Dr. \\'aller's Experiments with 

 Seeds.) 



Experiments such as these have enabled us to gain 

 some little insight into the complex processes involved 

 in the reaction of protoplasm to irritation, but still they 

 do not show us how it is that increased anabolic and 

 katabolic processes are set going subsequent to irrita- 

 tion. That metabolism produces electrical changes is 

 well known, and such changes are, of course, the .sequel 

 of any chemical and many physical processes; living 



