PHYSIOLOGY 



199 



As a result of the chemical processes involved in the decomposing 

 activity of assimilation, only the special end-product and one by- 

 product are at present known. Sachs discovered that the organic 

 compound, first to be detected as the special ultimate products of 

 assimilation in the higher plants, is a CARBOHYDRATE, which may either 

 remain in solution, or in the form of STARCH GRAINS may become 

 microscopically visible at the points of its formation. In the case of 

 the lower plants, in the Algae, for example, the first visible product is 

 often not starch but a fatty oil. 



A short time after assimilation begins, in sunshine, sometimes 

 within five minutes, minute starch grains appear either in the centre or 

 on the margins of the chloroplasts. These grains gradually enlarge until, 

 finally, they may greatly exceed the original size of the chloroplasts. 

 Should, however, the assimilation cease, which it regularly does at 

 night, then the starch grains are dissolved and as soluble carbo- 

 hydrates (glucose, etc.) pass out of the cell. In some plants (many 

 Monocotyledons) there is no starch 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 guard-cells of Monocotyledons. This seems then to be 

 a reserve substance rather than a special product of assimilation. In 

 Tropaeolwm, for instance, the formation of cane- 

 sugar precedes the production of starch in the 

 chloroplasts. 



The formation of starch may be shown to be 

 a direct result of assimilation by means of the 

 " iodine reaction," and without the aid of a 

 microscope. If a leaf cut from a plant pre- 

 viously kept in the dark until the starch 

 already formed in the leaves has become ex- 

 hausted, be treated with a solution of iodine 

 after being first discoloured in hot alcohol, it 

 will in a short time assume a yellowish brown 

 colour, while a leaf vigorously assimilating in 

 the light will, with the same treatment, take 

 a blue-black colour. In Fig. 182 the result 

 of the iodine, reaction is shown on a leaf, 

 part of which had been covered with a strip 

 of dark paper or tinfoil. The cells darkened 

 by the overlying paper or foil formed no 

 starch, while those exposed to the light are 

 shown by the iodine reaction to be full of 

 it. A green leaf kept in air devoid of car- 

 bonic acid, although fully exposed to the light, will similarly form 

 no starch. 



Fig. 182.— A leaf showing the 

 iodine reaction. Part of an 

 assimilating leaf was covered 

 with a strip of tinfoil. After- 

 wards, when treated with a 

 solution of iodine, the part 

 of the leaf darkened by the 

 overlying tinfoil, having 

 formed no starch, gave no 

 colour reaction. (f nat. 

 size.) 



