PHYSIOLOGY 187 



constant changes of the living substance, both transformations and 

 excretions. Even when the supply of food ceases these processes 

 continue, so that the death of the organism from starvation must 

 ensue if the substances that are used up are not at least replaced 

 in the process of nutrition. 



The Constituents of the Substance of the Plant. By means of 

 chemical analysis the constituent substances of plants can be most 

 accurately ascertained. It requires, however, no analysis to realise 

 that a part, often indeed the greater part, of the weight of a plant 

 is derived from the water with which the whole plant is permeated. 

 Water not only fills the cavities of living, full-developed cells, but it 

 is also present in the protoplasm, cell walls, and all organised 

 structures. By drying at a temperature of 110- 120 C. all water 

 may be expelled from vegetable tissues, and the solid matter of 

 the plant will alone remain. The amount of dried substance in 

 plants varies according to the nature and variety of the plant and of 

 the particular organ. In woody parts it constitutes up to 50 per cent 

 of their weight, but in herbaceous plants amounts to only 20 or 30 

 per cent. In succulent plants and fruits it makes up only 5-15 per 

 cent of their total weight ; in water-plants and Algae, 2-5 per cent, 

 while all the rest is water. 



The dried substance of plants is combustible, and consists of 

 organic compounds, which contain but little oxygen, and are con- 

 verted by combustion into simple inorganic compounds, for the most 

 part into carbonic acid and water. The elements CARBON, HYDROGEN, 

 and OXYGEN form the chief constituents of the combustible dried 

 substance. Next to them in quantity is NITROGEN, which is derived 

 principally from the protoplasm. After combustion of the dried 

 substance of plants there always remains an incombustible residue, 

 the ASH, consisting of the mineral substances contained in the plant. 

 As these mineral substances undergo transformation during the 

 process of combustion, they are found in the ash in chemical com- 

 binations different from those in living plants. From numerous 

 analyses made of the ash of a great variety of plants, it has been 

 determined that nearly all the elements, even the rarer ones, may 

 be present in plants. 



In addition to the four already named, the elements met with in the ash of 

 plants are sulphur, phosphorus, chlorine, iodine, bromine, fluorine, selenium, 

 tellurium, arsenic (which may be combined as a superphosphate in the soil), 

 antimony, silicon, tin, titanium, boron, potassium, sodium, lithium, rubidium, 

 calcium, strontium, barium, magnesium, zinc, copper, silver, mercury, lead, 

 aluminium, thallium, chromium, manganese, iron, cobalt, and nickel. 



Many of these elements, indeed, occur only occasionally and 

 accidentally, while others sulphur, phosphorus, chlorine, silicon, 

 potassium, sodium, calcium, magnesium, and iron are met with in 



