106 WATER-ABSORPTION AND TRANSPIRATION. 



during the process of burning; the nitrogen, as well as the 

 carbon and hydrogen, disappears into the air when the plant is 

 burnt. If the dried substance is charred instead of being 

 entirely burnt, we get charcoal, and if we collected the carbon 

 dioxide given off in complete combustion we should find that 

 a large percentage of the dry weight is represented by carbon 

 as much as one half. 



The incombustible residue, or ash, represents the mineral 

 substances, but in the process of burning these undergo 

 changes, so that they occur in the ash in chemical combina- 

 tions different from those present in the fresh plant-body. 



139. Analysis of Plant Substance. To make an exact 

 chemical analysis of plant- substance requires considerable 

 skill in qualitative and quantitative methods of chemistry. 

 But it is fairly easy to get a rough idea of the composition 

 of plants by simple experiments. 



* (a) Weigh some freshly-picked leaves and record the weight. Let 

 them lie in a dry place until they become apparently dry ("air-dry ") 

 and weigh again. From your work on transpiration you will know 

 that the loss of weight is due to loss of water. 



* (6) Dry the leaves in an oven and weigh them twice or thrice until 

 they show no further loss of weight. In this way you get the percentage 

 weights of water and of dried substance. As a check or " control," heat 

 some other " air-dry " leaves gently in a test-tube and note the deposi- 

 tion of water in the cooler upper part of the tube : this represents the 

 " hygroscopic " water which is driven out by heating. 



* (c) Dry and weigh a porcelain crucible, place in it the oven-dried 

 leaves, and heat thoroughly. The dry material chars and may flame 

 for a time while burning ; in ten minutes or so it is reduced to fine ash. 

 Find the weight of the ash, weighing the crucible and its contents 

 twice or thrice until no further loss occurs. To ascertain roughly the 

 amount of carbon, heat some weighed oven-dried leaves in a weighed 

 crucible, after covering them with a weighed quantity of dry sand ; 

 after about ten minutes' heating turn out the contents and find the 

 weight of the charcoal. 



14O. The Ash Constituents. The amount of the ash 

 increases with the age of the plant (why?). It also varies 

 in the different parts of the same plant at different stages in 

 its growth and life -history. 



The following table gives, for several common plants, the 

 approximate percentage weights of (1) the ash in the plant's 



