1220 THE TEMPERATURE FACTOR CHAP. 31 



the inside of the sturdy cell walls; this position may enable the chloro- 

 plast to sustain without injury a pressure that would destroy the chloro- 

 plasts freely suspended in the protoplasm. 



Whatever the macroscopic and microscopic cause of frost injury to 

 plant cells, the submicroscopic phenomenon is probably always a change in 

 the colloidal structure of the cytoplasm or of the chloroplasts. It is a 

 fundamental fact of life that the protoplasm colloids become granulated or 

 coagulated under the influence of mechanical forces. Mechanical stresses, 

 rather than direct temperature effects, probably explain the destruction of 

 cells by freezing. This is why dried cells, in which no ice formation is 

 possible, can sustain much lower temperatures than the same cells while 

 they still contain water. (These questions are discussed in the book of 

 Lepesclikin 1924.) 



One immediate effect of cooling below about 10° C. is a rapid increase in 

 viscosity and decrease in permeability of protoplasm. This change must 

 impede the diffusion of carbon dioxide to the chloroplasts, as well as the 

 translocation of the intermediates and products of photosynthesis. It can 

 thus contribute to the rapid decline of the photosynthetic efficiency, which 

 most plants experience in this temperature region. 



2. Optimum and Upper Temperature Limits; Heat Injury 



Above the lower temperature limit, the rate of photosynthesis increases 

 with temperature, first rapidly, then more slowly, until an optimum is 

 reached, followed by a rapid decrease to zero. For land plants in moder- 

 ate climates, the optimum is situated at 30-35° C. However, it lies much 

 lower for land plants and algae adapted to low temperatures, and much 

 higher for "thermophihc" algae that live in hot springs, and for tropical 

 desert plants. Some authors, e. g., Henrici (1921), Lundegardh (1924). 

 Ehrke (1929, 1931) and Stocker (1927, 1935), have observed temperature 

 curves with several (two or three) maxima. Even if these results (to be dis- 

 cussed in more detail in section C) are correct (which is doubtful), the oc- 

 currence of several temperature optima certainly is an exception and not 

 the rule. 



The decline of the net gas exchange (P — R) at high temperatures (which 

 eventually leads to a change in sign) is partly caused by a continued rapid 

 rise in respiration. (The latter increases exponentially up to 45 to 50° C.) 

 However, even if correction is made for enhanced respiration, the true 

 photosynthesis (P) also is found to possess an optimum, even though it is 

 less sharp, and is situated at a higher temperature, than the maximum of 

 net oxygen liberation (c/. fig. 31.2). 



