358 THE FOOD OF PLANTS 



since intense light ultimately exercises an injurious influence, there must be 

 a definite specific optimal intensity of illumination for each plant, although 

 if light acts directly upon other protoplastic functions as well, the general 

 optimum may not precisely coincide with the optimum for the assimilation 

 of carbon dioxide. Even for light-loving (photophilic} plants bright diffuse 

 daylight seems as a general rule to be preferable to strong sunlight. 



In nature plants are exposed to an illumination which varies according 

 to the season, and to the habitat and the time of day, while the different 

 leaves of a plant do not usually receive the same amounts of light. Thus 

 the innermost leaves of a horse-chestnut or beech may receive only a hun- 

 dredth part of the light which falls upon the fully exposed outer ones, 

 whereas in the open foliage of the birch, all the leaves probably obtain 

 sufficient light for active photosynthetic assimilation. In very weak light 

 leaves and branches become feeble and die, and this tends to thin out 

 excessively thick foliage to a certain extent 1 . 



The feeblest intensity of light at which growth is possible varies 

 very much in different plants, and- is dependent not merely upon the 

 assimilatory activity but also upon the amount of organic material consumed 

 in respiration as well as upon other factors. 



Rapid growth is generally accompanied by active respiration, and 



hence slowly developing plants are able to increase in dry weight upon 



.a smaller production of organic material no matter whether the feeble total 



assimilation is due to a diminution in the amount of chlorophyll present, 



or to a shady habitat. 



Respiration seems to be only moderately active in shade plants, while 

 the abundance of chlorophyll ensures the utilization of as much as possible 

 of the enfeebled light 2 . Many plants are able to grow indifferently either 

 in deep shade or in strong light, but typical shade plants are unable to 

 withstand prolonged exposure to intense illumination. On the other hand, 

 a lessened power of growth and feebler respiratory activity are usually 

 connected with the weaker assimilatory powers of certain photophilic 

 plants, as is the case in fleshy plants and in crustaccous Lichens 3 , for the 

 activity with which carbon dioxide is decomposed may surpass its rate of 

 production by respiration only under strong illumination. The same is 

 often the case in actively respiring young leaves, and Ewart has shown 

 that the attainment of the power of evolving oxygen in the light is not 



1 \Viesner, Sitzungsb. d. \Vien. Akad., 1895, Bd. civ, Abth. i, p. 605 ; Ber. d. Hot. Ges., 1894, 

 p. 78; \Viesner, Denkschr. d. Wien. Akad., 1896, Bd. LXIV, p. 73. 



" A. Meyer, Versnchsst., 1892, Bd. XL, p. 212. Cf. Haberlandt, Jahrb. f. wiss. Bot., 1882, 

 Bd. xni, p. 170; Spencer le Moore, Bot. Jahresb., 1888, p. 660. 



3 Jumelle, Rtv. gin. d. Bot., 1892, T. IV, p. in. Cf. Ewart, Journ. Linn. Soc., 1896, Vol. xxxi, 

 p. 381. On the illumination of a room, cf. Sect. 59. 



