August 22, 1907] 



NA TURE 



431 



systematic purposes. It may be, and ven- probably is, 

 optimistic to lool<; forward to the time when we shall know 

 uT/'iy the character is good in one, and worthless in another, 

 alliance. But when we do, I am convinced that the reason 

 will be found to lie in chemical and physical causes. We 

 are %-ery ignorant as yet of the details, but we can never- 

 theless even now form a fair guess at their general nature. 



In this connection I would venture to express the opinion 

 that much real harm is done by the toleration of an 

 uncritical habit of mind, all too common, as to the 

 significance of structures which are regarded as adaptive 

 responses to stimuli of various sorts. It is not enough to 

 explain the appearance of a structure on the ground of its 

 utility; properlv speaking, such attempts, so far from pro- 

 viding anv explanation, actually tend to bar the way of 

 inquiry just where scientific investigation ought to com- 

 mence. 



That many of the responses to such stimuli are of a 

 kind to render the organism " adapted " to its environ- 

 ment no one, of course, will dispute ; but to put forward 

 the adaptedness as an explanation of the process is both 

 unscientific and superficial. The size and the spherical 

 shape of duckshot are admirably adapted to the purposes 

 for which duckshot is used ; but this affords no insight 

 into the necessary sequence of cause and effect, which 

 makes the melted lead assume the characters in question 

 as it falls down the shot-tower. 



But manv people still find consolation and satisfaction 

 in an anthropomorphic and somewhat slipshod application 

 of a kind of doctrine of free-will to matters that really 

 call for rigorous examination into the causes which, under 

 given conditions, must inevitably and of necessity bring 

 about their definite result. 



One of the commonest responses to the stimulus of 

 wounding in the higher plants is the formation of a layer 

 of cork over the injured and exposed tissue. No one can 

 deny that this is a reaction of great utility, checking as 

 it does the undue evaporation of water and the entrance 

 of other parasitic organisms. And yet I suppose that no 

 one would go so far as seriously to maintain that the 

 obviousness of these advantages satisfactorily explains why 

 the cork layer is produced. It seems to me that an 

 investigation of the real underlying conditions which govern 

 such a modified reaction would be of immense value, and 

 that the information we might gain therefrom as to the 

 nature of the chemical processes involved would prove to 

 be of first-rate importance in tracking to their sources 

 some of the factors that influence the course of carbo- 

 hydrate metabolism within the cell, .\gain, we know how 

 easv it is to produce colour-changes in the leaves of certain 

 plants — e.g., rhubarb — by severing the vascular bundles, 

 and thereby interfering with the process of translocation. 

 Overton has shown how the accumulation of soluble carbo- 

 hydrates within the leaf of such a plant as Hydrocharis 

 modifies the metabolic processes within the cells. Thus in 

 bright light, under conditions of cold sufficient to arrest 

 starch formation, but not enough to stop photosynthesis, 

 a red-coloured substance makes its appearance in the cell, 

 and this' again disappears on raising the temperature, so 

 that the accumulation of soluble carbohvdrates diminishes. 

 The red colour which is associated with the change may 

 nossiblv by absorbing the heat ray aid in restoring meta- 

 bolism to its '* normal " course ; but such a teleological 

 explanation is not of general application, and gives no 

 real insight into the nature of the processes involved. 

 The well-known laboratory method, which we owe to 

 Klcbs, of inducing Eurotium to enter on a sexual phase 

 by keeping it at a temperature of 26° C. is another 

 example of the same order. The particular reaction that 

 occurs in each of these instances is that which necessarily 

 results under the specified conditions, and no other course 

 of chemical change is possible. 



In the last-mentioned example, Eurotium acts in a way 

 similar to that of drought, only the result is more ouicklv 

 produced. This perhaps indicates that we are dealin-^ 

 with a definite series of chanj?es which are inhibited bv 

 the presence of too much available nutriment supplied at 

 a temperature too low to enable it to be sufficiently rapidly 

 altered within the organism so as to give rise to the 

 specific substance, which is more directlv responsible for 

 the ascogonial phase of the life-history. Something of an 



NO. TQ73, VOL. 7 61 



analogous character is probably effective in the formation 

 of "fairy-rings," so typical of the growth of certain 

 agarics. This appearance of fairy-rings may be easily re- 

 produced in artificial cultures of moulds by appropriate 

 means. Thus if the nutriment agar be Ivcpt fairly dry, 

 so that the rate of diffusion of soluble materials is slowed 

 down, it is found that concentric zones of sterile and 

 sporiferous hvphge regularlv alternate with each other. An 

 explanation of this behaviour, which seems most probable, 

 is that the hyphae, after they have been growing over the 

 substratum for a certain distance, have acquired sufficient 

 raw material to provide for the building-up of the sub- 

 stance wHiich stimulates spore-production. When this has 

 taken place the substance so elaborated is used up and 

 spore-production ceases until a fresh supply of material, 

 under the conditions of the experiment, has been formed 

 to act in its turn as a new stimulus. This suggestion is 

 supported by the interference with the circular form of 

 zones that can be brought about by artificially interfering 

 with the rate of diffusion of the supply of nutriment in 

 the jelly. The rhythmical alternation of sterile and fertile 

 zones seems to prove that quantity of elaborated material 

 is an essential factor in the process, just as in the stimula- 

 tion of a motile organ the stimulus itself has to reach a 

 certain minimal intensity in order to cause a movement. 



The parallelism between the nutritive, i.e., the chemical, 

 stimulus in the case of the fungus and the minimal time- 

 stimulus required to provoke gcotropic movement is very 

 striking. For it will be remembered that there is evidence 

 in the latter instance also of the occurrence of a definite 

 chemical change as the result of the disturbance of normal 

 gravitational relations. This finds expression in the 

 accumulation of homogentisinic acid as the result of the 

 formation of an anti-oxidative substance which arrests the 

 complete disruption of tyrosin in the cells. Whether this 

 is the immediate cause of the geotropic movement, or 

 merely a concomitant of it, we cannot settle at present. 

 But it is of the highest interest to know that chemical 

 change is initiated as a result of the external gravitational 

 impulse, even when the latter is of too short duration to 

 produce an actual geotropic movement. .And although we 

 mav not at present be able to identify the exact material 

 which is directly concerned in these stimulatory or form- 

 ative processes, we have, as it seems to me, irresistible 

 evidence in favour of its real existence. It is more than 

 mere analogy that leads us to believe that the various 

 kinds of galls, for example, that may be formed on an oak 

 leaf owe their formation to the specific interference of the 

 secretion of the grub with the higher metabolic processes 

 going on in the cells of the leaf. 



I have alluded to the different conditions under which 

 given reagents may interact, and these may in turn very 

 materially affect the final result by modifying the course 

 of the reaction itself. We are coming to realise the fact 

 that the physical conditions of the cellular constiluents 

 exercise an important influence on the course of chemical 

 activity manifested within their range. We all know what 

 an important part water plays in ordinary chemical re- 

 actions, but the water question assumes a special promin- 

 ence when the reactions are going on in a colloidal matrix, 

 or rather in a mixture of colloids, such as the various 

 proteins that occur in the cell. Questions of rates of 

 diffusion, physical absorption, and the like have to be 

 taken into account ;. and beyond all these there remain the 

 series of remarkable electrical relations which the proteins 

 exhibit, as w'ell as those changes in surface-tension that 

 are, in part at least, connected with them. 



It is impossible to resist the belief that a closer study 

 of the physico-chemical changes that accompany a nuclear 

 division will yet throw much light on the mechanics of 

 this wonderful process. Indeed, we already possess some 

 data which are serving as starting-points for further in- 

 vestigation, and thev have placed some of the known facts 

 in a very suggestive light. 



It has often been urged as a reproach against the 

 histological methods employed in the study of the cell that 

 all such investigations can, after all, only give information 

 as to the character of coagulations or precipitations. Of 

 course this is perfectly true ; but provided we have 

 sufficiently good grounds for enabling us to feel confident 

 that the precipitation or coagulation faithfully maps out 



