100 PLANT PHYSIOLOGY 



sunlight and after three or four hours test their leaves for the 

 presence of starch as follows: Remove a leaf, immerse it in 

 hot alcohol for a few minutes to extract the chlorophyll and then 

 cover with a strong solution of iodine which will color the leaf 

 blue or not according as the starch is present or absent. To 

 avoid rupture of the sealing by the expanding air it is well to 

 use a bell jar with an opening at the top into which is placed a 

 cork through- which a glass tube passes. This tube should be 

 bent so that its other end is immersed in a dish of mercury. 

 As the air expands it passes out through this tube and escapes 

 through the mercury but the air and carbon dioxide from out- 

 side cannot enter. 



(g) On a large leaf of geranium (Pelargonium), or other 

 plant which produces starch in abundance in its leaves, clamp 

 on the upper side a flat cork and on the lower side a little box 

 (a wooden box such as cover glasses come in will be satis- 

 factory) blackened inside and whose sides 

 have been pierced from the outside by 

 numerous small holes running obliquely 

 away from the leaf. These holes admit air 

 (and COz) but as they point away from the 

 leaf any light admitted through them is ab- 



FIG. 45. Disap- , , J . . , , , , . .. , , 



pearance of starch sorbed by the blackened inner surface of the 

 box. Set the plant in the sunlight for sev- 

 eral hours then remove the leaf and treat 

 with alcohol and iodine as in (/). The spot protected from 

 light by the cork and the little box will show no starch. 

 To clamp two corks together on both sides of the leaf is un- 

 satisfactory, as in that case not only is the light cut off but the 

 C0 2 as well, so that the reason for the lack of starch in that case 

 is two fold. 



(h) Reserve carbohydrate in the form of starch may be 

 demonstrated in the tubers of potatoes, root of sweet potatoes 

 (Ipomoea batatas'), seeds of corn (Zea mays), wheat, beans, etc. 

 In the form of cane sugar it is present in the root of the beet 

 (especially in the sugar beet) , in the stem of corn and sugar cane, 

 etc. As hemicellulose it is present in the wood of mulberry 

 (Morus) and elm where it may be demonstrated by treating a 

 section with sulphuric acid followed by iodine solution. Food is 

 stored up in the seeds of cotton, castor bean (Ricinus), flax, 

 etc., and in the scales of onions, leaves of cabbage, etc., as fats. 



