98 PHOTOSYNTHESIS AND RESPIRATION. 



and even then the more deeply situated tissues would receive 

 very little energy as compared with the outer tissues. 



We saw that seedlings grown in darkness lose in dry 

 weight (Art. 65). In an experiment the dry weight of 

 100 Wheat grains was found to be 4 grammes, that of 

 50 Peas 11 grammes. In each case a batch of seeds, equal 

 in number and practically equal in weight, was germinated 

 in darkness, and in three weeks the dry weight of the 100 

 Wheat seedlings was found to be 1*5 grammes, that of the 

 50 Pea seedlings 6 grammes. 



The intensity of respiration varies greatly in different 

 species, in different plants of the same species, and in the 

 same plant at different stages of its life. Oily plants and 

 shade-loving plants respire feebly ; flowers and opening buds, 

 as well as germinating seeds, respire much more actively than 

 fully-grown roots, leaves, etc. 



At about 15 C. the following plants absorb, per gramme of their 

 fresh weight, the following volumes of oxygen in cubic centimetres : 

 Cacti, 3 to 25 ; Stonecrop, 70 ; Spruce Fir, 44 ; Broad Bean, 96 ; 

 Wheat, nearly 300. 



At 15 C. the volumes of carbon dioxide produced by the following, 

 per gramme of dry weight, are: Mustard seeds, 50 c.cs. ; Poppy 

 seeds, 120 c.cs. ; Lilac buds, 35 c.cs. ; Lime-tree buds, 70 c.cs. 



The spadix of Arum (Cuckoo-pint) was found to use up during its 

 opening 700 c.cs. of oxygen in twenty-four hours, the volumes (in c.c.) 

 used in the first six hours being 50, 70, 100, 140, 80, 40. A gramme of 

 spadix substance may give out as much as 30 c.cs. of carbon dioxide 

 in an hour, and half of the stored substance (starch and sugar) may be 

 used up in a few hours. 



The volumes of oxygen used up in twenty-four hours by the repro- 

 ductive parts of flowers (anthers and pistil), taking the volume of 

 these parts as unity, was found to be : Wallflower, 18 ; Tropaeolum, 

 16 ; Cucumber (male), 11 ; Cucumber (female), 7. 



The volumes of oxygen used by the leaves (in darkness) were (volume 

 of leaf = 1) : Wallflower, 4; Tropaeolum, 8; Passionflower, 5; 

 Holly, 0*9 ; Aspidistra, 0'5 (which, doubtless, largely explains why 

 this plant withstands unfavourable conditions indoors). 



133. Respiration affected by Temperature. Respira- 

 tion is not affected much by light, either by its presence or 

 absence, or its intensity, but heat is of great importance. 

 The optimum temperature for respiration is nearer to the 

 maximum than is the case with other vital processes. In 

 germinating seeds, 10 to 20 times as much carbon dioxide is 



