SECT. II 



PHYSIOLOGY 



219 



very conimoi), 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 neAv 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 demon- 

 strating 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 " 

 will appear blue on a light ground, as 



in Fig. 195, when the leaf after being 

 illuminated is treated with iodine. 



Fig. 195. — Assimilation experiment 

 with the leaf of Arioijsis peltata 



In some plants (many Monocotyle- (reduced). 

 dons) no starch is formed in the 



chloroplasts, but the products of assimilation pass in a dissolved 

 state directly into the cell sap. In exceptional cases, however, starch 

 is also formed where there is a surplus of glucose, sugar, and other 

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

 fruits. Tlie guard-cells of the stomata and the cells of the root-cap 

 of these Monocotyledons also contain stai'ch. In other cases only 

 a fraction of the product of assimilation appears as starch (in Helianthus, 

 for example, only J) 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 

 floatins: leaves on a sugar solution of suitable concentration. This 

 shows that the formation of starch does not stand in direct connection 

 Avith 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. 



Quantity of the Assimilate (^-■')- — If a leaf is exposed throughout 



