136 AMYLOGENESIS [CH. 



evolved) in a given time is by no means always the same, 

 even in leaves of the same plant, and it differs considerably 

 in different plants. 



Experiments have also shown that, under the same 

 conditions otherwise, a given leaf decomposes different 

 quantities of carbon-dioxide in a given time according to 

 the temperature, the amount of carbon-dioxide in the 

 atmosphere, and the intensity of the light. 



Each plant has, in fact, an optimum temperature for 

 assimilation, beyond or below which it decomposes less 

 carbon-dioxide per unit of surface in the unit of time ; 

 and similarly with the intensity of light, and the quantity 

 of carbon-dioxide in the air. 



Under certain circumstances, it is possible to make a 

 leaf decompose more carbon-dioxide by adding an excess 

 of that gas to the atmosphere, especially if the intensity 

 of the light is increased at the same time ; but the carbon- 

 dioxide must not exceed 8 to 10 per cent., or the process of 

 assimilation ceases owing to the injurious reaction on all 

 the life-processes, and even these quantities which of 

 course are relatively enormous, since the atmosphere only 

 contains '03 to *04 per cent, as a rule can only be 

 endured for a short time and in a bright light. 



The best proof of increase of weight by assimilation 

 in the leaves, however, is the following, the principle of 

 which was discovered by Sachs. 



A stencil-plate, with a figure, letter, &c., cut out in it, 

 is laid on the upper surface of a leaf of a Vine, Cabbage, 

 or other ordinary plant early in the morning of a bright 

 warm day, before sunrise, and the sun is allowed to shine 

 directly on the surface thus treated for a few hours : then 

 the leaf is cut off, and is at once plunged into boiling water 

 for a few seconds, and decolorised in warm alcohol. When 

 all the chlorophyll has been thus extracted, the white or 



