IHV. II 



PHYSIOLOGY 



253 



will appear blue on a light ground, as in Fig. 249, when the leaf after being 

 illuminated is treated with iodine. Instead of a stencil a suitable photographic 

 negative can be used, as MOLISCH has shown ; after illumination and subsequent 

 treatment with iodine a positive photograph is obtained (Fig. 250). 



In some plants (many Monocotyledons) no starch is formed in the chloroplasts, 

 but the products of assimilation pass in a dissolved state directly into the cell 

 sap. Starch is formed, however, where there is a surplus of glucose, sugar, and 

 other substances, as, for example, in the coloured plastids of flowers and fruits. 

 The guard cells of the stomata and the cells of the root-cap of these Monocoty- 

 ledons also contain starch. In other cases only a fraction of the product of 

 assimilation appears as starch (in Relianthus, for example, only ), while the rest 

 remains as sugar or is otherwise 

 made use of. It is thus clear that 

 the amount of starch formed cannot 

 always be taken as*a measure of 

 the assimilation. 



Starch formation can be induced 

 to take place in the dark by float- 

 ing leaves on a sugar solution ot 

 suitable concentration. This shows 

 that the formation of starch does 



FIG. -_>49. Assimilation experiment 

 with the leaf of Arivpsis peltata. 

 (Reduced.) 



'. The positive photoyraph obtained by cover- 

 ing a leaf of Tropaeolum which has been freed of 

 starch by the negative and exposing it to the sun. 

 After assimilation the leaf has been treated with 

 iodine. (After MOLISCH.) 



not stand in direct connection with the assimilation of carbon dioxide but is only 

 the result of the accumulation of sugar in the cell. 



In some Algae neither sugar nor starch but other products of assimilation are 

 formed, e.g. Floridean starch. 



The nature of the "fat-drops" which frequently appear in assimilating cells 

 and their connection with this process is still uncertain ( 32 ). 



The Quantity of the Assimilated Material depends on the one 

 hand upon the kind of plant and on the other upon the external 

 conditions to which it has been exposed. It can be said that a square 

 metre of leaf of an actively assimilating plant under optimal external 

 conditions produces between 0'5 and 1 gramme of dry substance per 

 hour. When it is considered how many square metres of leaf surface 

 are daily assimilating, a conception can be formed of the huge produc- 

 tion of organic substance in this largest of all chemical factories. 

 SCHRODER estimates the amount formed annually by land plants 



