244 BOTANY PART i 



Heat produced by Respiration. Respiration is, chemically and 

 physically considered, a process of oxidation or combustion, and, like 

 them, is accompanied by an evolution of heat. That this evolution 

 of heat by plants is not perceptible is due to the fact that consider- 

 able quantities of heat are rendered latent by transpiration, so that 

 transpiring plants are usually cooler than their environment ; and 

 also to the fact that plants possess very large radiating surfaces in 

 proportion to their mass. 



The spontaneous evolution of heat is easily shown experimentally, if transpira- 

 tion and the loss of heat by radiation are prevented and vigorously respiring plants 

 are selected. Germinating seeds (Peas), if examined in large quantities, show under 

 proper conditions a rise in temperature of 2 C. The greatest spontaneous evolu- 

 tion of heat manifested by plants has been observed in the inflorescence of the 

 Araceae, in which the temperature was increased by energetic respiration 10, 15, 

 and even 20 C. Also in the large flower of the Victoria reyia temperature 

 variations of 15 C. have been shown to be due to respiration. One gramme of the 

 spadix substance of an Aroid exhales, in one hour, up to 30 cubic centimetres CO 2 ; 

 and half of the dry substance (the reserve sugar and starch) may be consumed in a 

 few hours as the result of such vigorous respiration. In the process of wound - 

 healing in plants a noticeable rise in temperature also occurs. 



That other processes, in addition to respiration, co-operate in the production 

 of heat is apparent from the fact that the amount of heat evolved does not vary 

 proportionally to the carbonic acid exhaled. 



The Movement of Gases in Respiration 



In plants of simple construction the cells which are in direct 

 contact with the air or water can absorb the requisite oxygen 

 directly ; while cells in the midst of tissues are dependent upon the 

 oxygen which can diffuse through the surrounding cells. Such a 

 diffusion from cell to cell would not, however, be adequate, in the case 

 of the large cellular bodies of the higher plants, to provide the living 

 cells of the interior with a sufficient supply of oxygen. This is 

 accomplished by means of the air-spaces, which, as INTERCELLULAR 

 PASSAGES, penetrate the tissues in all directions and so bring to the 

 protoplasm of the inner cells the air entering through the STOMATA 

 and LENTICELS. The path of the respiratory gases is thus the 

 same as that followed by the gases in transpiration and assimilation 

 (pp. 205, 221). 



The movement of the gases within the intercellular spaces is due partly to the 

 diffusion, induced by the constant interchange of gases caused by respiration, 

 assimilation, and transpiration, and partly to movements in mass arising chiefly 

 from modifications of the temperature, pressure and moisture of the surrounding 

 atmosphere, but also increased by the movement of the plants themselves, through 

 the action of the wind. 



That the intercellular spaces were in direct communication with each other and 



