526 TRANSFORMATION OF ENERGY 



Just as in the case of every machine, so in the case of the organism, several con- 

 ditions must be fulfilled if its various functions are to be carried out satisfactorily. 

 In addition to internal factors there are also a number of external or formal con- 

 ditions (Pfeffer, Physiol. II, 76) which must be taken into consideration, the 

 significance of some of which is quite obvious, although the part played by others 

 is as yet far from being understood. The need for water and certain constructive 

 material is apparent on the face of it ; oxygen also is required in order that 

 respiration may be carried out, a physiological process without which, as a general 

 rule, no movements can take place. In addition to these material conditions, 

 heat and light must be taken into account, for a definite amount of heat, a de- 

 finite degree of temperature, is one of the most fundamental formal conditions 

 of plant life, while some, though by no means all reflex movements require 

 light of appropriate intensity for their manifestation. Finally, injurious 

 external influences must be absent, such as poisons and narcotics, which either 

 merely delay the response or prevent it taking place altogether, or even may 

 bring about the death of the organism, according to the degree of concentration 

 in which they occur. Every insufficiency in any of the formal conditions tends 

 to bring on a state of rigor, cold rigor, heat rigor, drought rigor, &c., and each of 

 these rigors, if long continued, tends to become fatal. 



None of these facts are new to us ; we have summarized them once more 

 merely that we may add to them a few general remarks. Let us inquire first 

 as to the significance of the intensity of these factors. We have abundant data 

 at our disposal on the subject, for in almost all vital processes we have estab- 

 lished the fact that the influence of formal conditions is, in the most intimate 

 way, dependent on their intensity, and that that dependence may be expressed 

 graphically by means of a curve in which three cardinal points may be recog- 

 nized, viz. a minimum, an optimum, and a maximum. It is often said that this 

 dependence is limited to organisms, but that is by no means the case. It must 

 be remembered that in the inorganic world also ' optima ' may be recognized (com- 

 pare Errera, 1896). Since water attains its maximum density at 4" C, we may 

 express this dependence of density on temperature by means of a curve which 

 exhibits one optimum point, but which certainly shows no minimum or maximum. 

 There are also, however, processes, entirely apart from those occurring in or- 

 ganisms, which exhibit well marked minima, optima, and maxima in their relation 

 to temperature. Thus the solubility of sodium sulphate has its minimum at o°C., 

 its maximum at 100°, and its optimum at 33° C. Betol (compare Tamman, 1898) 

 reminds one even more of the organism, for it melts at 96° C. and, after cooling, 

 remains liquid as long as it is maintained above + 25° C. and under — 5° C. Tem- 

 peratures above the minimum (— 5°C.) and below the maximum ( +25° C.) do 

 not act in the same way, for at 10° C. we reach an optimum, inasmuch as far more 

 crystals make their appearance at that than at other temperatures. 



Temperature influences the various processes in the plant in a variety of 

 different ways ; the graphic curve for assimilation is quite different from that 

 for respiration or for growth, and we cannot be wrong in assuming that what 

 is true of these processes is true of others. Naturally the behaviour of different 

 organisms varies to a much greater extent still. Take the case, for example, of 

 the dependence of responsive movements on oxygen only ; we cannot wonder 

 that typical anaerobes require no oxygen and may even be brought into a state 

 of rigor by its presence, while the same pathological condition is induced in 

 aerobes by its absence. It is a very remarkable fact, however, that among 

 genuine aerobes, also, very great variations are met with in relation to the 

 minimum percentage of oxygen required ; Correns (1892) has shown that 

 at least 6 per cent, of the normal amount of oxygen present in the atmosphere 

 must be present before Passiflora can carry out haptotropic movement, while 

 Mimosa leaves and the tentacles of Drosera will respond to contact stimulus 

 when oxygen is entirely absent. The various phases in the reflex action also 



