WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 169 



223. Microscopical Detection of Pliotosyiithetic 

 Starch. To detect the presence of the small starch grains 

 formed by photosynthesis in the chloroplasts of assimi- 

 lating cells e.g. the mesophyll-cells and the guard-cells of 

 leaves cut vertical or tangential sections of the leaves if 

 thick, or mount entire thin leaves, and treat by one of the 

 following methods. 



(1) If the tissue is very transparent, simply treat with 

 iodine solution, and note that the starch grains are asso- 

 ciated with the chloroplasts. 



(2) Place the leaf in hot alcohol, until decolorised ; then 

 place it in potash solution ; rinse in water, treat with dilute 

 acetic acid to neutralise the potash ; rinse again in water ; 

 then treat with iodine and mount in water. 



(3) Another method is to make up Schimper's chloral- 

 hydrate-iodine, by dissolving chloral hydrate crystals in 

 as much water as will cover them, and then adding to the 

 solution a little iodine tincture ; on placing this solution 

 on a thin leaf, the chlorophyll is dissolved, the tissue be- 

 comes transparent, and the starch grains swell up and are 

 stained blue. 



224. The Quantity of the Fhotosynthate, or carbo- 

 hydrate product of photosynthesis, may be roughly deter- 

 mined as follows. Choose a pot plant with a large number 

 of leaves or leaflets of about equal size, e.g. Fuchsia, Broad 

 Bean, or Clover ; or use a Tropaeolum plant, in which the 

 area of the roughly circular leaf can be estimated by mea- 

 suring the radius. Place the plant in darkness till the 

 leaves are starch-free. 



Eemove a number of the leaves, before exposing the 

 plant to light; kill them (to prevent loss of photosyiithate by 

 respiration) by holding them, impaled on a mounted needle, 

 in the steam issuing from a kettle or in the upper part of 

 a test-tube containing water boiled over a Buiisen flame ; 

 dry them in an oven until on weighing them no further loss 

 in weight occurs, and determine their dry weight. Expose 

 the plant to light for a few hours ; then remove an equal 

 number of leaves, treat them in the same way, and find 

 their dry weight. The difference will give a rough idea of 



