470 MANUAL OF BOTANY 



cesses which we have discussed in the precedmg chapter, and these 

 coincidently place at the disposal of the plant a certam store of 

 energy which, originally derived from the radiant energy of 

 light, has been transformed from the kinetic to the potential 

 condition, and which is rendered again kinetic hj the catabolic 

 changes. So constant is this relation that it may be said that 

 heat is always evolved where and while oxygen is being 

 absorbed. 



Though respiration is always proceeding wherever there is hv- 

 ing protoplasm, the activity of the process is modified by different 

 ph^'sical conditions. The degree of illumination of most plants is 

 continually varying, and though respiration does not depend upon 

 light, its activity is not the same under all conditions of the 

 latter. We have already noticed that variations of temperature 

 affect differently the absorption of oxygen and the exhalation of 

 CO... The absorption of oxygen is suspended if the tempera- 

 ture is too low, it increases as the latter rises to a certain 

 optimum point, which varies with different plants, and at higher 

 temperatures it progressively decreases. The exhalation of CO., 

 is smaller at low temperatures than the absorption of oxygen, 

 but is greater at higher ones. 



The process of respiration is also affected to a considerable 

 extent by the nature of the substances which serve as nutritive 

 material for the reconstruction of the protoplasm. It has 

 already been pointed out that seeds containing oil absorb more 

 oxygen during germination than those whose reserve material is 

 largely starch. Organs which contain much proteid matter respire 

 more copiously than others which contain but little. The nature 

 of the inorganic salts absorbed also influences the process to a 

 certain extent. 



CHAPTER XII. 



GROWTH. 



In studying the growth of plants we must consider the relation 

 which it bears to the processes of metabolism which we have 

 akeady discussed. AYe have seen that the constructive metabo- 

 lism or anabolism is much greater than the destructive, or cata- 

 bolism. There results from this a considerable mcrease in the 

 substance of the plant, as well as an accumulation of potential 

 energy which can be made available by the plant by the destruc- 



