218 



HOT ANY 



I'AKT I 



not from photosynthesis but from the oxidation of ammonia into nitrous acid, and 

 this into nitric acid (chemosynthesis). The formation of organic substance in tin 1 

 sulphur bacteria and especially in the so-called purple bacteria is also insufficiently 

 understood, but photosynthesis appears to play a part in it. 



As a result of the chemical processes involved in the decomposing 

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

 product are at present known. SACHS discovered that the or^nn it- 

 compound, first to be detected as the special ultimate product 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 a 

 number of plants (e.g. the Algae) the first visible product is often not 

 starch but a fatty oil, protein, or some other secondary product. 



A short time after assimilation begins, 

 in sunshine, sometimes within five minutes, 

 minute starch grains appear in the chloro- 

 plasts. 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 carbohydrates (glucose, etc.) 

 pass out of the cell. In some plants (many 

 Monocotyledons) there is no starch formed in 

 the chloroplasts, but the products of assimila- 

 tion pass in a dissolved state directly into the 

 FIG. 199. A leaf : showing the cellsap. In exceptional cases, however, starch is 



iodine reaction. Part of an a ] go f orme( ] where there IS a Surplus of glllCOSe, 

 assimilatini' leaf was covered -, , , f > 



with a striper tinfoil. After- sugar, Jind other substances, as, for example, m 



wards, when treated with a the guard Cells of the StOTTiata of MoilOCOtvle- 



soiution of iodine, the part dons, and in the coloured plastids of flowers and 



of the leaf darkened by the . . T.I i r j.- 



overlying tinfoil, having fruits. In other cases also only a fraction ot 

 formed no starch, gave no the product of assimilation appears as starch 

 colour reaction. (} nat, ( in Hdianthus, for example, only J) while the 

 carbohydrates formed in the first place (sol- 

 uble hexoses, especially dextrose C (! H 12 6 ) are in part transformed 

 into di- and poly-saccharides. In a chemical sense the process of 

 assimilation is thus an asymmetrical one. 



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 previously kept in the 

 dark until the starch already formed in the leaves has been removed 

 in a soluble form, be treated with a solution of iodine after being 

 first decolorised 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 owing 



