62 EXPERIMENTAL PLANT PHYSIOLOGY 



It has thus been proved: firstly, that the starch 

 formed in the leaf is converted into sugar ; and, secondly, 

 that it is possible to convert starch into sugar by 

 boiling it with a mineral acid. 



Now it is quite evident that the starch present in the 

 leaf is not naturally hydrolysed by a mineral acid in 

 this way. The hydrolysing agent in the case of the 

 leaf is a ferment termed " diastase." This diastase can 

 be extracted from the leaf, but the operation is beyond 

 the scope of this book. 



The action of diastase differs from that of a mineral 

 acid in that it is able to effect the change at the 

 ordinary temperature. 



The soluble sugar into which insoluble starch has 

 now been converted travels down the stem, and some 

 of it is afterwards reconverted into starch, in which form 

 it is stored as a reserve-food in tubers and bulbs and 

 other such structures. 



Eventually the starch and sugar formed in the plant, 

 together with the substances taken up in solution from 

 the soil, are used by the plant for the building up of its 

 solid framework and for the formation of the living 

 substance, the protoplasm, contained within it. 



Experiment 41 



Aim. — ^To determine where the food material, manu- 

 factured in the leaf, passes through the stem. 



Method. — A ring of tissue about an inch wide and 

 reaching as far as the wood, is removed from the stem 

 of a branch of a tree or from the main stem of a young 

 potted seedling- tree. A two-year-old sycamore answers 

 well. 



The removal of this ring of tissue, consisting of 

 bast, cortex, and bark (page 18), does not inter- 

 fere with the passage of the watery solutions up 

 the stem, since they ascend through the wood only 

 (Experiment 8). 



Observations. — The following observations were made 



