604 PHYSIOLOGY. 



in equilibrium throughout its whole structure. The action of one force 

 producing results in one direction is compensated for by the action 

 of other forces producing results in other directions. The general 

 result is a series of compromises of a character necessarily varying 

 according to the relative intensity of this or that action. To dis- 

 entangle these mixed results, and to ascertain what are the peculiar 

 effects produced by each agency, first separately, and then in con- 

 juction with others, is the task of the physicist, provided with appa- 

 ratus and instruments of precision. 



Effect of Heat. Vegetable life is not carried on below the freezing-point 

 of water, nor, as a rule, above 50 C., although some seeds and the minute 

 germs of Cryptogams appear to be capable of retaining their vitality 

 under much more elevated temperatures. Each plant, each part of a 

 plant, and each function performed by it, is carried on best and most fully 

 at a certain degree of temperature ; it is checked by excess in either direc- 

 tion. The limits within which healthy action and growth take place are 

 necessarily very different in different cases. The germination of seeds, the 

 production of flowers, the ripening of fruits, all the processes of life de- 

 pend upon the degree of temperature. The successful cultivation of plants 

 in the field, in the garden, in the hothouse depends on the knowledge and 

 practical application of this fact and of the cognate ones derived from a 

 knowledge of the effects of other agencies. Attempts have been made 

 to observe the temperature required for the germination of seeds, the pro- 

 duction of leaves, flowers, &c. Thus, for the ripening of Wheat, a certain 

 aggregate amount of heat is required from the period of germination to that 

 of ripening. Other things being equal and within certain limits, it is im- 

 material whether that heat be received in a shorter or longer period. In 

 southern countries the crops ripen more quickly than in England, in Eng- 

 land than in Scotland ; but the aggregate amount of heat required is about 

 the same in all cases, though diffused over a longer period in the one case 

 than the other. Vegetation in the extreme north is often effected with 

 great rapidity ; for, although the summer is short, the amount of heat is 

 great, and the duration of exposure to light, during the growing period, 

 longer than in more southerly latitudes ; hence it is possible to cultivate 

 in the open air in the north of Norway Australian annual plants. 



The lowest temperature at which the grains of chlorophyll turn green, 

 according to Sachs, is, for French Beans and Maize between 6 and 15 C. ; 

 for Brassica Napus above 6 C. ; for Pinus pinea between 7 and 11 C. 

 The highest temperature at which leaves already formed and still yellow 

 turn green was, according to the same observer, above 33 C. for the 

 first-named plants, and above 36 C. in the case of the Onion. 



Exhalation of oxygen and assimilation have been observed "by Bous- 

 singault, in the case of the Larch, at temperatures only just above the 

 freezing-point. What amount of heat puts a stop to the process has not 

 been ascertained. True respiration or elimination of carbonic dioxide 

 takes place, according to Deherain, Wolkoff, and Mayer, at a temperature 

 considerably below that which is requisite for growth, being earned on 

 even below the freezing-point, and being manifested in direct proportion 

 to the increase of temperature until a certain maximum is obtained, a 

 maximum exceeding that whereat growth becomes arrested. 



