676 TRANSACTIONS OF SECTION K. 
that really call for rigorous examination into the causes which, under given con- 
ditions, 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 to seriously 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 ag 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 carbohydrate metabolism within 
the cell. Again, we know how easy it is to produce colour-changes in the leaves 
of certain plants—eg., rhubarb—by severing the vascular bundles, and thereby 
interfering with the process of translocation. Overton has shown how the 
accumulation of soluble carbohydrates 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 carbohydrates diminishes. The red colour which is associated with the 
change may possibly by absorbing the heat ray aid in restoring metabolism to 
its ‘normal’ course; but such a teleological explanation is not of general applica- 
tion, and gives no real insight into the nature of the processes involved. The 
well-known laboratory method, which we owe to Klebs, 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 caused 
by drought, only the result is more quickly produced. This perhaps indicates that 
we are dealing with a definite series of changes which are inhibited by 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 directly responsible for the ascogonial phase 
of the life-history. Something of an 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 reproduced in artificial cultures of moulds by 
appropriate means. Thus if the nutriment agar be kept fairly dry, so that the rate 
of diffusion of soluble materials is slowed down, it is found that concentric zones of 
sterile and sporiferous hyphz regularly alternate with each other. An explana- 
tion of this behaviour, which seems most probable, is that the hyphe, 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 substance which 
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 asa 
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 stimulation 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, ¢.e., the chemical, stimulus in the case of 
the fungus and the minimal time-stimulus required to provoke geotropic move- 
ment 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 
