[Chap. XVIII RESPIRATION AND PLANT DEVELOPMENT 165 



because respiration consumes a much larger percentage of the food 

 made by them in such situations. 



Equally illuminating is the fact that the compensation point varies 

 greatly with the kind of plant. In Scotch pine it is reported to occur at 

 18° -20° C. when the intensity of light is 1/25 that of the full sunlight 

 of summer. Comparing this report with the data obtained by several 

 other investigators, we find that the compensation point occurs in some 

 of our pines, arbor vitae, and tamarack at about this same light intensity. 

 For white pine and red oak it is nearer 1/50 to 1/70, and for beech and 

 sugar maple, 1/100 to 1/200. For a few of the herbs, ferns, and mosses 

 that grow in dense shade, it may be as low as 1/300; and for some algae, 

 1/10,000 or less. In general it is lower in "shade plants" than in "sun 

 plants." This difference is less pronounced at low than at high tem- 

 peratures. 



There can be little doubt that the differences in respiration and 

 photosynthesis in plants indicated by the compensation point are an 

 important factor in the survival and distribution of plants in different 

 habitats in relation to temperature and hght. Beech and sugar maple 

 endure more shade than oaks, and many oaks endure more shade than 

 most pines. One kind of forest therefore may gradually replace another 

 by producing more shade than its seedlings can endure. On lawns and 

 campuses many examples of differences in shade endurance by plants 

 may be observed. For instance, during a period of excessively hot clear 

 days bluegrass is favored by a small amount of shade, but it endures 

 more shade in the cool days of early spring and late autumn than it 

 does in the hot days of midsummer. 



Comparative rates of respiration. The rate of respiration is greatest 

 where there is rapid growth, as in germinating seeds and opening 

 flowers. In some plant organs respiration may be. more rapid on the basis 

 of weight than in animals. The amount of food oxidized during a day 

 ( == 1500 Cal. ) by a man at rest is equal to about 4 per cent of his 

 dry weight. The amount of food oxidized in a corn plant each day is 

 equal to about 1 per cent of its dry weight. The corresponding figure for 

 an opening cluster of flowers is about 8 per cent; and for some kinds of 

 germinating seeds, about 20 per cent. The lowest rates of respiration 

 occur in dry seeds and other dormant structures, and there is compara- 

 tively little respiration in woody stems and other hard parts in which 

 there are only a few living cells. 



The amount of carbon dioxide liberated by the respiration of the 



