180 BOTANY part i 



Sub-Section I 

 METABOLISM (^) 



I. The Chemical Composition of the Plant {^) 



Any consideration of the metabolic changes in the ^^lant requires 

 a knowledge of its chemical composition. This is studied by chemical 

 methods. 



Water and Dry Substance. — Some insight into the composition of 

 the plant can l)e obtained without special means of investigation. 

 Every one who has dried plants for a herbarium knows that the plant 

 consists of water and dry substance. He also knows how the removal 

 of the water influences such fundamental physical properties of the 

 plant as its rigiditj^ and elasticity. By means of Aveighing it is 

 easy to show how large is the proportion of water in the total weight 

 of the plant. For this purpose it is not sufficient to expose the plant 

 to the air, for Avhen air-dried it still retains a considerable proportion 

 of water, which must l)e removed by drying in a desiccator or at a 

 temperature of over 100^ C. It can thus be ascertained that the 

 proportion of water is very considerable ; in woody parts some 50 per 

 cent, in juicy herbs 70-80 per cent, in succulent plants and fruits 

 85-95 per cent, and in aquatic plants, especially Algae, 95-98 per cent, 

 of the weight of the plant consists of water. 



Ash. — While we can thus distinguish by drying between the water 

 and the dry substance of the plant, we are able by burning to dis- 

 tinguish between the combustible or organic material and the incom- 

 bustible substance or ash. The fact that the plant leaves an ash is 

 evident in the burning of wood or in the smoking of a cigar ; the 

 microscope further shows that even minute fragments of cell-wall or 

 starch grains leave an ash on burning. Information as to the quantita- 

 tive relations of the ash is afforded by analysis, which shows especially 

 that the various organs of a plant differ in this respect ; leaves, for 

 example, tend to contain more than stems. It has thus been found 

 that the dry substance of the leaves of Brasdca rapa contains about 

 20 per cent of ash, Avhile the stems have only 10 per cent. 



The constituents of the ash also vary according to the nature of 

 the soil and other external influences. On the other hand, distinct 

 species may accumulate different quantities of mineral substances, even 

 when exposed to the same external conditions. 



While the majority of the more common elements occurring in 

 the earth are found in the ash of plants, only a few elements are 

 present in sufficient amount to be quantitatively estimated. These 



