RESPIRATION 129 



the living substance. The resulting decompositions, 

 though taking at first the same course as in other cases, 

 did not go so far. Instead of the liberation of carbon 

 dioxide, there was found a considerable increase in the 

 amount of certain organic acids, chiefly malic and oxalic 

 acids, which remained in the cells, and which probably 

 represented the ultimate products of the decomposi- 

 tions. 



We have mentioned in the earlier part of this chapter 

 that there are a few lowly forms of plants which do not 

 require oxygen for their vital processes, and which do not 

 absorb it. Many of these so-called anaerobic plants are 

 killed by exposure to this gas. We must not, however, 

 necessarily conclude that their metabolism is of a totally 

 different kind from that of others, but rather that they set 

 up the decomposition and reconstruction of their proto- 

 plasm in a different way from those plants which need a 

 supply of oxygen to determine them. We shall return to 

 this point in a subsequent chapter, in which we shall 

 consider also the meaning of the complex metabolic 

 processes, \vhich we have so far discussed. 



Though respiration is always proceeding wherever 

 there is living protoplasm, the activity of the process is 

 modified by different physical conditions. Of these, 

 temperature is one of the most important. There is a 

 lower limit, beyond which it appears to be suspended, 

 though life is not destroyed. This limit varies in different 

 plants, but is generally one or two degrees below the 

 freezing point of water. In a few cases, stwft as Conifers 

 and Lichens, it may % eVen be --10 C., but this is rare. 

 As the temperature rises from tnis minimum point, the 

 activity of respiration increases up to a certain optimum 

 point, which is usually not well defined, and which varies 

 considerably in different plants. If the temperature is 

 raised only a little higher than this, the living substance is 

 rapidly injured, and its respiration is checked. Variations 



