193 LECTURE XII. 



between the. freezing-point of'wilter, on the one hand, and about go°C. on the 

 other, indicate those intensities of heat-motion at which plant life generally is still 

 possible. It is quite conceivable, and even occurs occasionally, that certain phe- 

 nomena of vegetation may still occur even below the freezing-point of water, because 

 from various causes the water contained in the cells only begins to crystallise at 

 a few degrees below zero. However, these are isolated cases : in the great majority 

 the vital movements in general only begin at a few or several degrees above the 

 freezing-point. On the other hand, the temperature of 50° C. also denotes the 

 upper limit only quite generally, since the majority of the vegetative processes are 

 brought to. a standstill below this temperature ; and temperatures of 45° to 50° 

 persisting for a long time are simply fatal to most plants. It depends here essentially 

 upon the abundance of water or the succulence of the parts of the plant concerned 

 whether they freeze' at lower temperatures, or perish at too high a temperature 

 below or above 50° C. Parts containing little water, as the majority of ripe, dry seeds 

 and winter buds, are exceedingly resistent even to low degrees of cold, and the 

 former even resist temperatures of 60° to 80° for a long time; whereas very succulent 

 organs often freeze even at temperatures in the neighbourhood of the freezing- 

 point (this, however, is a specific peculiarity of certain plants), and are killed at 50° 

 or even under. 



Within the specified range of temperatures there lies for each individual vegeta- 

 tive function a certain degree of temperature at which it developes its greatest 

 activity. We call this the optimum temperature ; thus distinguishing that point of the 

 thermometer at which any one vegetative phenonlenon exhibits its maximum 

 activity. At every degree of temperature above or below this optimum, the vegetative 

 phenomenon in question will be less active. Exact investigation shows also 

 that, proceeding from the lower hmit of temperature, at each higher constant degree 

 of temperature already mentioned, the activity of the phenomena of vegetation is 

 greater, untU at the specific optimum temperature itself the maximum activity is 

 attained; and if the temperatures rise still higher, the activity decreases: step by step, 

 until, on attaining the upper limit of temperature, it ceases entirely. If this high 

 temperature acts for a short time only, the vegetative phenomenon may be renewed 

 when a lower temperature recurs ; if, however, the high temperature mentioned 

 has continued too long, death ensues. 



In this case especially, where it is a matter of the dependence of the life of the 

 plant upon the temperature, we may again take the example of the steam-engine 

 already employed. We may compare the lower zero-point with that condition 

 where the tension of the steam just suffices to overcome the friction of the machine, 

 and to bring about a slow movement, which with increasing tension of the steam 



' Concerning the freezing of plants, cp. Sachs, • Krystallbildungen bet dem Gefrieren und 

 VerSnderung dir Zellhaiite bei dem Aufthauen.saftiger Pflanzentheile,' in the 'Berecliten der kgl. 

 Sachs. Gesellsch. der Wiss.,' Febr. i86o, where I first described the separation of crystalline water 

 from cells. In the ' Untersuchungen uber das Erfrieren ,der PJlanzen ' in H. 5 of the periodical, 

 'Die landwirthschaftlichen Versuchsstationen ' (Dresden,- i860), I gave a theory of freezing on 

 ■which all more recent researches in this direction have been based. This vfas improved in my 

 ' Lehrbuch der Botanik^ (1868-74). Miiller (Thiirgau) made valuable further researches, 'Land- 

 wirthschaftliche Jahrbiicher,' pub. Thiel (Berlin, i88o). 



