EXTERNAL CONDITIONS OF LIFE. 191 



the individual phenomena ol' vegetation, a detailed study of the corresponding 

 structural relations of the organs. 



Much, however, may be stated generally with reference to the influence of ex- 

 ternal causes on the phenomena of life ; since here it is only a matter of proving, by 

 means of observation and experiment, what kinds of vital phenomena appear when 

 certain alterations are produced in the environment of the living plant. In this way 

 it has been shown, even in the still very imperfect condition of our knowledge in 

 this respect, that a far greater portion of the phenomena of life are called forth by 

 external influences than one formerly ventured to assume ; and that what the living 

 organism performs of itself, by reason of its inherited structure, always concerns only 

 the specific nature of the performance. Whether and with what intensity this takes 

 place depends, however, entirely upon the play of external influences. 



What I would here emphatically lay down, is the fact that every phenomenon 

 of life arises from two factors : on the one hand, from the structure transmitted 

 from the mother organism, and, on the other, from external forces working on this 

 structure. Every phenomenon of life is the product of these two factors, neither of 

 which is at all effective by itself. The one factor, the external influences, which we 

 may also distinguish as the external conditions of life, we will now take into view 

 somewhat more closely in its most general outlines. 



The life of the plant is a coherent chain of the most various motions of the 

 smallest particles, the atoms and molecules of its substance, from which arise in the 

 plant mass-motions of the entire organs, which, however, are slow and mostly difficult 

 to perceive. The most general and important source of vital force through which 

 the Hfe-motions in the bodies of plants are called forth is, however, Heat^ As 

 is well known, heat is to be imagined as a motion of the smallest particles of matter, 

 which is transmitted from one substance to another. The intensity of this motion, or 

 the force with which the smallest particles vibrate, is called the temperature, which 

 in its turn is measured by means of the thermometer. Experience shows now 

 that the vital motions in the interior of the plant do not occur until the tem- 

 perature of the environment, which by degrees becomes communicated to the plant, 

 reaches a certain height — a definite degree of the thermometer; i.e. the heat-motion 

 imparted to the plant must effect a certain intensity of the vibrations of its smallest 

 particles, in order that those further motions of the atoms and molecules may take 

 place by which the various chemical processes of nutrition, and all the various 

 molecular motions of which the Hfe of the plant consists, are called forth. 



Numerous, but by no means final researches^ have shown that temperatures 



' I have given a comprehensive description of the effects of heat on vegetation, with detailed 

 references to the literature up to the year 1865, in my 'Handbuch der Experimcntal-physiologie der 

 Pßanzen'' (Leipzig, 1865), pp. 47-68 ; and supplied more details concerning freezing in my 

 ' Lehrbuch der Botanik,' 1868, p. 562. Cp. also the succeeding editions of the latter book. 



- Concerning the upper and lower limits of temperature of vegetation, cp. my treatises in the 

 ' Regensburger Flora,' 1863 — 'Die von'ibergchettdett Star7-e-zustande periodisch beweglicher umi 

 reizbarer Pßajizenorgane,'' pp. 449, &c. ; and 'Flora,' 1864, 'Über die Temperaturgrenzen der 

 Vegetation' p. 8 ; and further, p. 497, ' Über den Einfluss der Temperatur auf das Ergrünen der 

 Blätter: More details in this connection are to be found in my treatise, 'Physiologische Unter- 

 suchungen über die Abhängigheit der Keimung von der Temperatur,' Jahrb. für wiss. Bot B. II, 

 1S60, p. 338. 



