DIV. ii .PHYSIOLOGY 221 



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, while the stems have only 10 per cent (cf. p. 238). 



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 

 are the non-metals Cl, S, P, Si, and the metals K, Na, Ca, Mg, 

 and Fe. 



Organic Substance. Chemical analysis is not needed to show 

 that the plant contains carbon in a combined form. Every burning 

 log or match shows by its charring that it contains carbon. The 

 examination of a piece of charcoal in which the finest structure of the 

 wood is retained, shows further how uniformly the carbon is distributed 

 in the plant, and how largely the substance of the plant consists of this 

 element. Accurate weighing has shown that carbon constitutes about 

 one-half of the dry weight of the plant. On combustion of the dry 

 plant the organic substance is changed, and passes off in the form of 

 carbon dioxide and water, ammonia or free nitrogen. It contained 

 the elements H, 0, N, and C chemically combined ; some of the 

 elements mentioned as occurring in the ash may also occur in organic 

 compounds. 



Source of the Materials. There are thus only the following 

 thirteen elements found in considerable quantity in the plant : 



H, Cl, 0, S, N, P, C, Si and Na, K, Mg, Ca, Fe. 



When the plant is growing their amount is continually increasing 

 in the plant, and they must therefore be continually absorbed from 

 without. 



As a rule, only gases and liquids can enter the plant; solid 

 substances have to be brought into solution before they can pass 

 through the firm cell walls. When, however, cell walls are absent, as 

 in the Flagellates and Myxomycetes, the naked protoplasm is able 

 to surround and thus to absorb solid particles. 



