THREE CARDINAL POINTS OF TEMPERATURE. 193 



becomes accelerated, until the machine works not only with a certain velocity, but 

 also in such a manner that all the individual movements work together in the most 

 advantageous manner ; this would correspond to the behaviour of the plant at the 

 optimum temperature. If the tension of the steam is increased by supplying more 

 heat to the machine, dangers arise with the increasing velocity of its motion : 

 individual parts are heated and expanded too much, and others are strained until 

 they rupture ; increased centrifugal force may cause the fly-wheel to break, and the 

 machine shordy becomes destroyed by its own motion. We may compare these 

 processes in a certain sense with the death of the plant from heat. 



By the establishment of the three cardinal points of temperature (v.'hich 

 expression includes the lower and higher temperatures as well as the optimum), 

 it is seen that each single vegetative phenomenon in any one species of plant 

 in general possesses its particular cardinal points. In other words, the lowest 

 temperature at which perceptible growth takes place, does not necessarily suffice 

 for the development of chlorophyll, or for assimilation, or for the irritability of 

 motile organs, and so forth ; and when this is determined for one species of plant, 

 the lower zero-points of these functions in another species are by no means necessarily 

 the same. This is likewise the case with reference to the optimum temperatures 

 and the upper limits of temperature. In general it is shown, however, that most 

 plants flourish best at certain medium temperatures, somewhere between 15° and 

 30° C, because within these limits the various phenomena of vegetation take their 

 course with sufficient energy and work harmoniously together. The diversity of the 

 lower zero-points of the various functions may, however, bring it about that at certain 

 lower temperatures, somewhere between 5° and 10° C, the various functions no 

 longer work harmoniously together, so that pathological conditions are induced. 

 It is observed, for instance, that in the spring time the young leaves of cereal 

 plants grow, it is true, but in spite of bright illumination they remain yellow, 

 because the lower limit of temperature for growth is not so high as that for the 

 development of chlorophyll. Similarly, it is occasionally observed during the 

 regularly recurrent cooling of the air about the middle of June, that plants which 

 require a relatively high temperature, e.g. Beans {Phaseolus), Maize, Gourd, 

 Buckwheat, etc., unfold new leaves, it is true, during such periods of cool weather, 

 but for the reasons given above they remain yellow, until at a higher temperature 

 the development of the chlorophyll is rendered possible. 



No object would be served by bringing forward here all the numbers which 

 various observers have to a certain extent established as to the limits of temperature 

 and the optima of the various vegetative phenomena. Better opportunities for this 

 will occur later on, when considering the individual phenomena of vegetation them- 

 selves. Yet it may in some measure contribute to the explanation of what has been 

 said hitherto if a few numbers at least are mentioned as examples. The growth of 

 the seedling in its dependence upon temperature has been most frequently observed', 



^ In my ueatise, 'Physiologische Untersuchtingen über die Abhängigkeit der Keimung von der 

 Temperatur; in the Jahrb. für Wiss. Bot. (i860), I first showed that the growth of roots and shoots 

 above an optimum temperature is again retarded, and may thus be represented by a curve which 

 first ascends from the abscissa and then returns to it ; ^vliile up to that time, even by Boussingault, 

 simple proportionality between temperature and growth had bi.cn asbumcd. That the form of a curve 



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