336 THE FOOD OF PLANTS 



colouration *. In such cases, as well as when the chloroplastid is completely 

 bleached by intense light, the power of assimilating carbon dioxide is 

 irrevocably lost, whereas the chloroplastids of Conifers which have become 

 brown in winter may turn green and recover the power of assimilation in 

 spring (Ewart, 1. c., p. 390). 



A transitory inhibition of the power of photosynthetic assimilation may 

 be produced, as Ewart has shown, by the action of the most varied injurious 

 agencies, which in more concentrated form ultimately cause death 2 . The 

 stoppage of the chlorophyll-function is to a certain extent a precursor of 

 death, but if the action of the injurious agency is not too prolonged recovery 

 may occur after a longer or shorter latent period. Chloroplastids may be 

 perfectly normal in colour and in external appearance when in this inactive 

 condition, and hence some obscure change in their internal constitution or 

 assimilatory mechanism must have been produced. In certain cases rapid 

 recovery is possible, but in others a prolonged period intervenes before the 

 normal conditions can be restored. Similar latent periods of recovery are 

 exhibited when plasma-streaming, irritability, growth, &c., have been 

 inhibited by the prolonged action of depressing external agencies. 



A weakening and ultimate cessation of the power of assimilating carbon dioxide 

 may be produced by exposure to extremes of temperature or to intense sunlight, 

 by the action of poisons such as carbonic acid, ether, chloroform, antipyrin, &c., 

 by partial asphyxiation, and by the accumulation of the assimilatory products (Ewart, 

 ' I.e.). Thus Ewart found that leaves off/ex, fiuxus, Prunus, &c., exposed for a short 

 time only to a temperature falling from o C to 4C or 6C immediately recom- 

 menced to assimilate at a normal temperature, whereas when exposed to the same 

 low temperature for a few days the chloroplastids became completely inactive, and 

 were able, if they remained living, to evolve oxygen in the light only after being kept 

 for from one to twenty-four hours at a normal temperature. It has not been found 

 possible to induce a permanent inhibition of this function without either killing the 

 chloroplastids or destroying the chlorophyll, nor have any normal chloroplastids 

 been as yet observed which have permanently lost all power of photosynthetic 

 assimilation (Sect. 52). The weakening in the power of assimilation which Bous- 

 singault noticed in leaves kept in an excess of carbon dioxide for some time 

 is simply a special example of assimilatory inhibition, and the cessation of the 

 chlorophyll-function in certain mosses and lichens, observed by Jumelle and 

 apparently supposed by him to be a permanent change, is another example of the 

 same phenomenon 3 . 



1 Kreusler,Landw.Jahrb., 1885, Bd.xiv, p-95i ; Engelmann,Bot.Zeitung, 1881, p-446 ; Ewart, 

 Joum. Linn. Soc., 1896, Vol. XXXI, p. 437. On changes iu the shape of the chloroplastids caused by 

 external agencies, cf.Klebs,Unters. a. d. Bot. Inst. z. Tubingen, 1 883, Bd.i, p. 268 ; Schimper, Jahrb. f. 

 wiss.Bot.,i885,Bd.LXVi,p. 194; Richter, Flora, 1892, p. 55 ; Ewart, I.e., pp. 449, 451,565, 567,569. 



2 Ewart, Jonrn. Linn. Soc., 1896, Vol. xxxi, p. 364 ; 1897, p. 554. An abstract of the above 

 papers is given by Pfeffer in Ber. d. Sachs. Ges. d. Wiss., 1896, p. 311. 



3 Boussingault, Agron., &c., 1868, T. IV, p. 287 ; Pringsheim, Sitzungsb. d. Berl. Akad., 1887, 

 p. 768. Cf. also Engelmann, Bot. Zeitung, 1888, p. 717 ; Jumelle, Rev. g<hi. d. Bot., T. IV, p. 263. 



