RESPIRATION.—ASSIMILATION.—METABOLISM. 823 
which the crude sap undergoes in the leaves and other green 
parts by the action of light and air have been already alluded 
to in treating of the Functions of Leaves ; it will be here, 
therefore, only necessary to state in what those changes es- 
sentially consist. They are:—I1st. The transpiration of the 
superfluous fluid of the crude sap in the form of watery vapour, 
by which it becomes thickened. 2nd. The taking up from the 
air of oxygen and giving off of carbon dioxide, small quantities 
of water being probably formed at the same time, to which the 
term Respiration is now applied. The oxygen thus taken up in 
respiration is necessary to the vitality of the protoplasm, as also 
. for the oxidation of nutrient matters during the process of 
metastasis, &c. Respiration is most evident during the night, 
for the large quantity of oxygen given off during the day in 
the process of assimilation completely obscures the former 
change. 3rd. The absorption and decomposition of carbon 
dioxide, by which carbon, that most important constituent of 
plants—is added to the crude sap, whilst oxygen is evolved, 
carbohydrates being at the same time produced. To this the 
term Assimilation is commonly applied. The carbohydrates 
so formed may be starch, fat, or cane sugar (sucrose), but more 
especially starch. A further process is found to take place in 
some of the assimilated substances; thus they may change 
their position, passing from the cells in which they were 
formed to others, generally also undergoing at the same time 
a change in their chemical composition ; which combined 
changes are termed metabolism or metastasis. The differences 
between assimilation and metabolism may be seen in the Potato, 
where by the former process starch is formed in the chiorophyll- 
bearing leaves, which in its turn is converted into a glucoside in 
the stem and branches, and back again into starch in the tubers 
by metastasis. The crude sap being thus altered, then contains 
in itself the various nitrogenous and non-nitrogenous matters 
which are required for the development of new tissues, and 
the formation of other organic products, which are commonly 
called secretions. It is then termed E!aborated Sap. 
Those organic matters which are necessary for development 
or growth are termed constructive materials, whereas those which 
are formed by metabolism or metastasis and which are not con- 
structive—may be divided into two groups— 
1. Degradation products, such as wood and cork, which can 
never be reconverted into constructive materials, though of the 
greatest use to the plant in giving mechanical support ; pro- 
tecting the internal living tissues from frost, enabling plants to 
withstand the scorching heat of the sun, and in other ways. 
Many gums, as tragacanth, gum arabic, and others; and gum 
resins, as myrrh and bdellium ; are alsc formed from the cell- 
walls, &c., of different plants, and are, therefore, other examples 
of such products. 
