PLASTIDS 



65 



senting the energy involved in terms of calories, the reactions may be 

 written as follows (Gortner) : 



Photosynthesis: 6CO2 + 6H2O + 677.2 calories = CeHiaOe + 6O2. 

 Respiration: CeHiaOe + 6O2 = 6CO2 + 6H2O + 677.2 calories. 



The carbohydrate first elaborated is presumably in solution and there- 

 fore invisible. In many plants this product either accumulates in the 

 dissolved state or is removed from the chloroplasts as 

 fast as it is formed. Very commonly, however, there 

 is a temporary excess, which is deposited as minute 

 granules of starch in the chloroplasts. This "assimila- 

 tion starch" is therefore the first visible product of 

 photosynthesis in most green plants; in some species 

 the product is evidently an oil. As Meyer originally 

 showed, the starch is actually synthesized within the 

 body of the chloroplast (Fig. 28, A). It is later trans- 

 formed through the agency of enzymes into some solu- 

 ble compound, usually a sugar, in which form it may 

 be carried to growing regions where, after further 

 changes, it is built into the structure of the plant, or it 

 may pass to storage organs where it is transformed 

 into "reserve" or "storage" starch. This deposition 

 of reserve starch is brought about through the agency 

 of amyloplasts, which are leucoplasts capable of chang- piastids. A, dividing 



1111,1 ■ J. • 1 u chloroplasts of Fu7i- 



mg already elaborated organic materials, such as ^^^^ ^j^j^ ^^^^^^ ^^ 



glucose, into starch (Fig. 28, C). assimilation starch. 



The statement made by Schimper (1880) and s^^XLTpiZtZTyg- 

 Meyer (1883, 1895) that starch is always formed by nema, with several 

 piastids still holds good in its essential feature: so far abo^^ a &e^ntrai^pyre- 

 as is certainly known, no primary product of photo- noid. (After Bour- 

 synthesis is formed in the cytoplasm apart from plas- copLt^ UmyiSpiitt)' 

 tids. The granules of "Floridean starch" in the red in tuber of Phajus 

 algse lie in the cytoplasm, but they begin to form at g7a'il'''^^f' reserve 

 the surface of the plastid as flat plates which become starch. {After Stras- 

 cup shaped or conical, with their flattened sides "'"^^''-^ 

 remaining in contact with the plastid as long as they continue to grow.^ 

 It is, however, reported that they are formed some distance away from 

 the piastids in some species (Mangenot, 1923a). If such is actually the 

 case, it is highly probable that they arise through the transformation of 

 a non-visible product (sugar?) of the photosynthetic activity of the pias- 

 tids, and are not immediately built up from water and carbon dioxide. A 

 similar interpretation may be placed upon corresponding appearances 

 reported in higher plants. 



^Henckel (1901\ Kylin (1913). 



Fig. 28.— Forma- 

 tion of starch by 



