STUDIES OF IRRITABILITY OF PLANTS PEIRCE 77 



cally food-manufacturing organs, dependent for their efficiency upon the 

 amounts of light and of carbon dioxide which can penetrate to the deeper as 

 well as more superficial cells. I cannot see that the revolution of a culture 

 in a covered crystallizing dish at no greater speed than four times a minute 

 could promote diffusion or otherwise increase the supply of carbon dioxide 

 sufficiently to account for the larger size of the prothalli and of the liver- 

 worts on the turn-tables, as compared with those motionless on the shelf. 

 The supply of carbon dioxide to the leaves of wheat seedlings revolving on 

 turn-tables may be greater than for the plants stationary on the shelf. But 

 the supply of light is certainly increased in the same way that the supply of 

 heat is increased for the man who first turns his face and afterwards his back 

 to a fire until he is comfortably warmed. 



DISCUSSION AND SUMMARY. 



1. An analysis of the influence of light upon growth in plants shows 

 that it affects the direction, kind, rate and amount of growth^^. Phototropic 

 bending of plants and plant-organs is of common and long-established 

 knowledge. That the kind of growth is influenced by light has been shown 

 mainly by the earlier ecologists, such as StahP*, and by the experi- 

 mental morphologists and physiologists like GoebeP*, Klebs^^, and 

 Vochting^" It has long been supposed that the rate of growth, and also 

 the amount, are greatly influenced by light, that, as Sachs^' would have it, 

 light depresses the rate of growth, other things being equal. Common ex- 

 perience shows that plants grown in darkness or in insufficient light are long, 

 slender, spindling, or at least "drawn," as compared with plants growing for 

 the same length of time, and under otherwise identical conditions, in the 

 light. But that the cause of this difference can be expressed in the usual 



12 Peirce, G. J. Text book of Plant Physiology, p. 210. 1903. - 



13 Stahl, F. Ueber den Einfluss des sonnigen oder schattigen Standortes auf 

 die Ausbildung der Laubblatter. Zeitschr. f. Naturwissenschaft, XVI. Review in 

 Botanische Zeitung, 41, 1883. 



1* Goebel, K. Einleitung in die experimentelle Morphologie der Pflanzen. 1908. 

 And the literature there cited. 



15 Klebs, G. Willkiirliche Entwickelungsanderungen bei Pflanzen. 1903. And 

 the literature there cited. 



16 Vochting, H. Ueber den Einfluss des Lichtes auf die Gestaltung und Anlage 

 der Bliiten. Jahrb. f. wiss. Bott. XXV. 1893. 



17 Sachs, J. Lectures on the Physiology of Plants. Oxford, 1887. Etc. 



