144 BOTANY 



rhythmically recurrent alteration of this kind in the root-cells concerned is responsible 

 for the flow of sap produced as the result of what is termed root pressure. The investiga- 

 tions of Adrian Brown have shown that grains of barley and other cereals possess in the 

 dead cell layers of the pericarp a membrane which is semipermeable, and in this respect 

 it recalls some of the physical properties of plasmatic membranes. If grains of barley 

 are steeped in dilute sulphuric acid for example the water is able to pass through and 

 swell the grain, but the acid is excluded. This will continue till a high degree of concen- 

 tration of acid is produced such as would at once prove fatal to the living cells of the 

 grain if they were directly exposed to its action. The membranes of certain fungi, e.g. 

 Penicillium, have long been known to possess this property of semipermeability in a strik- 

 ing degree, thus accounting for the frequent development of the mycelium in solutions 

 containing copper sulphate, sodium arsenite, and other poisonous substances. 



The effects of temperature are of importance in influencing the rate at which chemical 

 change occurs in the plant. They are complicated however by the invariable collateral 

 action of other factors. They have been observed to accord with expectation in experi- 

 ments which can be carried on in vitro, as for example in the study of enzyme action. 

 But within the organism the events are more complex. Attempts to investigate the 

 curious results of the effects of cold in retarding plants, and of a temporary immersion 

 in a hot bath for forcing them into premature flowering have not as yet yielded conclu- 

 sive results. Muller-Thurgau, who has recently investigated this subject, many years 

 ago ascertained the fact that cooling potatoes to about 3 2 F. led to the conversion of 

 starch reserves to sugar, and that on raising the temperature increased respiration fol- 

 lows, some of the sugar undergoes reconversion to starch, but other changes also go on 

 which render the potatoes unpalatable. Perhaps there is a resemblance here to the 

 modified chemical change which was observed some years ago by Overton in the leaves 

 of frog-bit (Hydrocharis) and other plants, when they were exposed to full light and low 

 temperature; that is, to conditions under which photosynthesis was possible, but under 

 which the means for the removal of the products of photosynthesis, either by transloca- 

 tion or by conversion to starch, were inadequate. The consequent abnormal concentra- 

 tion of sugars leads, under such circumstances, to the formation of glucosides and colour 

 bodies characteristic of individual species of plants. 



In plant metabolism important advances have been made by F. F. Blackman and 

 his school, particularly in the processes of respiration and photosynthesis. A more 

 complete analysis of the events included in photosynthesis has shown them to be more 

 complex than they were formerly thought to be, and that whereas it is pretty certain 

 that light is concerned in preliminary analytic effects, it is by no means so sure that it 

 determines the subsequent syntheses that culminate in the production of carbohydrates. 

 As a lateral extension of the study, particularly of the conditions that affect the met- 

 abolic processes here alluded to, a considerable clearing up has ensued of the obscurity 

 that lurked in the old terms optima and minima. It has become recognised that any 

 metabolic reaction (in the widest sense of the term) of plants is the net result of the inter- 

 action of a varying number of different factors. The word " optimum " therefore is 

 not only without definite meaning, but is actually misleading, inasmuch as if used in a 

 quantitative sense at all it entirely fails to recognise the limiting influence on a reaction 

 which is imposed by new conditions. Thus the quantity of carbon dioxide that can be 

 utilised in a given time by a given plant is not a necessarily constant quantity, but is 

 affected by conditions of temperature and of illumination, and these two factors are in 

 turn susceptible of modification in their results by others. 



Many of the so-called vital processes of cells have resolved themselves into reactions 

 that can be conducted in vitro, and especially is this true of reactions in which enzymes 

 take a part. Unfortunately no enzyme has been prepared as yet in the pure state. It 

 is always associated with extraneous, generally albuminous, matter, and it cannot be re- 

 garded as certain that each enzyme is to be considered as a specific and separable sub- 

 stance in the ordinary meaning of the word. The number of different enzyme actions 

 is very large, and is still increasing, and while it is true that so far as we know at present 



