IRRITABILITY 211 



the plant is adequately nourished. Light retards the 

 growth of those organs which do not depend upon it for 

 energy for food-manufacture. 



The form as well as the size of parts is greatly influenced 

 by light. If a branch of cactus, Opuntia, be potted and 

 kept in darkness, the buds will develop into branches which 

 are small and cylindrical, while the branches formed in the 

 light are larger and flat.* Marchantia ' gemmse, germinated 

 in the light but on a clinostat, develop in two months into 

 small erect tubular plants, wholly unlike the flat expanded 

 thalli which form when light and gravity act normally.! 



The daily periodicity of light and darkness are almost 

 coincident with the daily periodicity in growth-rate. This 

 was first shown by Sachs, J who devised machines (auxan- 

 ometers ) for autographically recording the growth of plants 

 during extended periods. These records show that the rate 

 of growth in length, of plants furnished with all the food 

 they need, will reach its maximum about sunrise, its mini- 

 mum about sunset. That growth is not most rapid when 

 there is least light, or slowest when the light is strongest, 

 shows that the effect of light, like that of gravity, is cumu- 

 lative (p. 204). Maximum growth-rate and minimum 

 temperature almost coincided in Sachs's experiments, but 

 this indicates, not that coolness favors growth, but that 

 Sachs did not make his experiments at constant tempera- 

 tures. Sachs's experiments, repeated at constant tempera- 

 tures, show that the maximum and minimum growth-rates 

 occur at approximately the same times as before, and that 

 light and not heat is the predominant influence in con- 

 trolling the rate of growth. 



It is claimed that light favors the growth of the vegeta- 

 tive organs of submersed aquatics. A certain minimum 



* Vochting, H. Uber die Bedeutung des Lichtes fiir die Gestaltung 

 blattformiger Kakteen. Jahrb. f. wise. Bot., Bd. 26, 1894. Goebel, K. 

 Organographie, I., pp. 211+, 1898. Also Flora. Bd. 80, 1895. 



f Czapek, F. 7. c., 1898. p. 261. 271, etc. 



J Sachs, J. von. Uber den Einfluss der Lufttemperatur und des Tag- 

 eslichtes auf die stiindlichen und taglichen Anderungen des Langenwachs- 

 thums (Streckung) der Internodien. Arb. d. Bot. Inst., Wiirzburg, 1872. 

 Gesamirelte Abhandlungen, Bd. II.. 1893. 



