158 Mr. W. Mnrton Holmes on the 



If we submit a dried plant to slow combustion, with a limited 

 supply of air, there remains a black residue, retaining in a great 

 measure the form of the plant. This residue is charcoal or 

 carbon, which in the living plant existed in combination with 

 hydrogen and oxygen. Had the combustion taken place with 

 a full supply of air so as to consume the carbon as well as the 

 hydrogen and oxygen, a white residue or ash would have re- 

 mained. The elements found in this ash, in a combined state, 

 are potassium, calcium, magnesium, sodium, iron, sulphur, 

 phosphorus, and chlorine, which are essential to nutrition. 

 Besides these other elements are found in the ash of some 

 plants, but are not considered to be essential to nutrition. The 

 organic compounds found in plants contain the elements carbon, 

 hydrogen, oxygen, nitrogen, and sulphur. 



A well-developed living cell consists of a firm elastic mem- 

 brane of cellulose closed on all sides, containing a layer of a soft 

 substance called protoplasm in close contact with it, and itself 

 enclosing a watery fluid called cell-sap. In most plants nume- 

 rous green granules, called chlorophyll-granules, are found em- 

 bedded in the protoplasm in those parts exposed to the light. 

 The method by which a cell is able to absorb nutriment depends 

 upon the phenomenon known as osmosis, or liquid diffusion, 

 which may be explained as follows : — Whenever two fluids of 

 different densities are separated by an organic membrane, there 

 will always be a tendency to equilibrium of density between the 

 two from the formation of a double current, one passing from the 

 lighter to the denser, the other from the denser to the lighter. 

 This process is repeated from cell to cell throughout the plant. 



Atmospheric air consists of a mixture of 77 parts by weight of 

 nitrogen and 23 parts of oxygen, with about 3 to 6 measures of car- 

 bonic acid in 10,000 measures of air, and also a trace of ammonia. 

 Those plants which contain the green chlorophyll absorb the 

 whole of their carbon, through numerous openings on the leaves 

 called stomata, from the carbonic acid of the air. Under the 

 influence of light the cells containing chlorophyll decompose the 

 carbonic acid, forming other compounds with less oxygen in 

 their composition, called carbohydrates, and the superfluous 

 oxygen is set free. Formerly this process was regarded as an 

 act of respiration, and was considered one of the characters by 

 which plants could be distinguished from animals, the former 

 absorbing carbonic acid and giving up oxygen, the latter ab- 

 sorbing oxygen and giving up carbonic acid. It is now known 

 that during respiration in plants carbonic acid is given off just 

 as in animals, but the giving off of oxygen during the process of 

 assimilation masks the action of respiration, which can only be 

 recognised when assimilation is at a standstill during the night. 

 During daylight the need of carbon is very great, and a fresh 



