56 CHLOEOSTS. [CH. IT 



exposed to light, but they do not become green ; while 

 control plants placed on wet filter-paper under a bell-jar 

 soon develope chlorophyll. It is not necessary to use 

 boiled water, the amount of air in ordinary spring water 

 being insufficient for the respiration of land-plants. 



(70) Seedlings in hydrogen. 



To demonstrate the fact in another way mustard seed- 

 lings may be placed in hydrogen. We use the L-shaped 

 vessels recommended by Detmer 1 . The difference between 

 the experimental seedlings and the control in air is clear 

 after 24 hours. The vessel may be filled with hydrogen 

 by displacement of water. 



(71) Iron. 



The effect of iron salts in restoring a green colour to 

 chlorotic 2 leaves, may be occasionally demonstrated on 

 chance specimens. A chlorotic branch of Robinia in the 

 Botanic Garden at Wiirzburg was restored to a healthy 

 green by screwing a funnel into the tree close to the base 

 of the branch, and pouring into it a solution of an iron 

 salt 3 . 



In the absence of chance material, chlorotic plants 

 must be produced by growing them, by the water culture 

 method, without iron. It is best to grow some five or 

 six iron-starved plants so as to have control plants and 

 to make sure of material for several experiments. The 

 addition of a few drops of a solution of iron chloride 

 should lead to the development of chlorophyll, but ac- 



1 Praktikum, p. 26. 2 See Sachs' Arbeiten, in. p. 433. 



3 Sachs' Vorlesitngen, p. 343. 



