24 ASSIMILATION. [CH. II 



water-plants (e.g. Elodea, Potamogeton) which have been 

 placed in the dark room for about four days. The 

 control-plants must be grown either out of doors or in 

 a greenhouse. 



(28) Effect of dull light. 



Sachs' method may be used to demonstrate a fact, 

 the knowledge of which is of practical value to the 

 physiologist 1 , namely, that plants in a laboratory suffer 

 from want of light far more than would be readily 

 supposed and that accordingly experimental plants can- 

 not be too carefully kept in the best light available. 



Choose two equally vigorous pots of clover, let one 

 remain in bright diffused light out of doors, and place the 

 other on a table in the middle of the laboratory. The 

 plant in the laboratory must be under a bell-jar on 

 account of the dryness of the air, and therefore to make 

 the control experiment fair the plant out of doors should 

 also be under a bell. After two days compare the amounts 

 of starch in the two plants. 



(29) Local effect. 



Various means may be used to convince oneself that 

 assimilation is confined to the illuminated regions of a 

 leaf. Part of a leaf may be darkened, while still attached 

 to the plant, by bending it down and burying the- apical 

 half in a flower-pot of finely sifted dry earth. The leaf 

 should be buried one day and examined in the afternoon 

 of the following day, taking care before the leaf is un- 

 covered to mark on it the depth to which it was buried. 

 1 See Detlefsen. Sachs' Arbeiten, m. p. 88. 



