252 BOTANY PART i 



considerably according to the temperature. At 15 C. a litre of water contains about 

 as much C0 2 as a litre of atmospheric air. The dissolved bicarbonates also play 

 an important part in the supply of carbon to aquatic plants ( 31 ). 



Artificially conducting carbonic acid through the water increases, to a certain 

 degree, the evolution of oxygen, and the assimilatory activity. Similarly an 

 artificial increase of carbonic acid in the air is followed by increased assimilation. 

 Whether and to what extent an artificial enriching of the air in C0 2 would be of 

 advantage in horticulture or agriculture cannot be said ( 32 ). 



The C0 2 assimilation, like all vital processes, is dependent on the 

 temperature. It begins at a temperature a little above zero, reaches 

 its maximum at about 37 C., and again stops at about 45 C. 



These cardinal points not only have different positions in different plants but 

 do not remain constant for any particular plant. This is especially true of the 

 optimum which in the course of a few hours may sink from 37 C. to 30 C. In bright 

 warm weather assimilation does not reach its full possible value since the supply of 

 carbon dioxide is then insufficient. 



Other less important factors need not be considered in detail. It may be 

 mentioned, however, that many substances can bring about a temporary, or 

 ultimately a permanent, limitation or arrest of the assimilatory process. 



Products of the Assimilation of Carbon Dioxide. It was assumed 

 above that sugar was formed from the carbon dioxide, and analysis 

 in fact shows that the amount of sugar in a foliage leaf is increased 

 after exposure to sunlight. It is true that grape-sugar is neither 

 always nor only shown to be present ; usually other more complex 

 carbohydrates appear. These can all, however, be traced back to 

 hexoses like grape-sugar, and arise by the union of two or more 

 molecules of hexose and the loss of the elements of water. Prominent 

 among them are cane-sugar (C 12 H 22 O n ) and starch (C 6 H 10 5 ) n . The 

 occurrence of starch in the chloroplasts of illuminated foliage leaves is 

 very common, but by no means general. When the leaves are placed 

 in darkness for some time the starch disappears. When on the other 

 hand a part of the plant from which the starch has been removed is 

 exposed to sunlight, new starch grains often form in the chloroplasts 

 in a surprisingly short time (5 minutes) ; these soon increase in size 

 and ultimately exceed in amount the substance of the chloroplast itself. 

 Since starch is stained blue by iodine the commencement of assimila- 

 tion can be readily demonstrated macroscopically (SACHS* method). 



Leaves which have been in the light have their green colour removed by means 

 of alcohol, and are treated with a solution of iodine ; they take on a blue colour. 

 If the amount of starch is greater the colour is a deeper blue or almost black. The 

 depth of the coloration thus affords a certain amount of information as to the 

 quantity of starch present. To demonstrate smaller amounts of starch the 

 decolorised leaves are placed, before staining with iodine, in a solution of potash or 

 of chloral hydrate in order to swell the starch grains. This method of demonstrat- 

 ing assimilation can also be used to show that the starch only appears in the 

 illuminated portions of the leaf. If a stencil of opaque material from which, for 

 instance, the word "Starke " has been cut is laid on the leaf, the word " Starke " 



