182 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



part of the sugar has migrated and been used up in 

 respiration. 



Keep an Onion plant in darkness for two days ; then cut 

 out parts of the leaves, measure the area of the pieces, kill 

 with steam and dry them, and record their dry weight. 

 Expose the plant to light, and after several hours take an 

 equal area of leaf, and in the same way find its dry-weight. 

 Note that increase in weight occurs in starchless as well as 

 in starch-forming leaves as the result of photosynthesis. 



241. Starchy and Non- starchy Leaves. Experi- 

 ments have shown that on the whole the variations in the 

 capacity for producing starch as a photosynthesis product 

 are characteristic of certain families. Very large quan- 

 tities of starch occur in the leaves of Solanaceae and 

 Papilionaceae ; large quantities in Papaveraceae, Crassu- 

 laceae, Geraniaceae, Labiatae, etc. ; moderate amounts in 

 Caryophyllaceae, Ranunculaceae, etc. ; very little in many 

 G-entianaceae and Iridaceae ; and none at all in Allium, 

 Scilla, and various other Liliaceae, also in many Amarylli- 

 daceae and Orchidaceae. 



Even in the plants that are richest in starch, it can be 

 proved that starch is not the first product of photosyn- 

 thesis ; when water-plants are exposed to light, the giving- 

 off of oxygen (which accompanies the assimilation of carbon 

 dioxide) begins almost instantly, though starch only 

 appears after an interval of several minutes. If a starch - 

 leaved plant (e.g. Sunflower or Bean) is kept in darkness 

 until sugar, as well as starch, has disappeared from the 

 leaves, and the plant is placed in the light, it is found that 

 the appearance of starch is preceded by that of sugar. 



The difference between starchless leaves (e.g. Onion) and 

 starchy leaves simply arises from the fact that in the 

 former the sugar produced is stored as such, while in the 

 latter the sugar produced is partly converted into starch. 

 It is easy to prove that even in normally starchless leaves 

 starch may be produced if sugar is present in sufficient con- 

 centration, which can be attained by (1) separating the 

 leaves from the stem, and thus preventing the translocation 

 of the sugar ; (2) increasing the amount of carbon dioxide 



