108 THE BIOLOGY OF A PLANT. 



poses of repair or growth, it may be re-converted into starch 

 after journeying as glucose through the plant, and be laid down 

 as " reserve starch" in the parenchyma of the rhizome, or else- 

 where. When this reserve supply is finally needed at any point 

 in the plant, it is again changed to glucose and transported 

 thither. It is probable that new leaves and new tissues gen- 

 erally, are always formed in part from this reserve starch, and 

 not solely from newly-formed starch. 



The Outgo. The outgo, like the income, is of two kinds, mat- 

 ter and energy, but it cannot be so readily tabulated. 



The plant suffers annually a great loss both of matter and of 

 potential energy in the production of spores and in the autumnal 

 dying-down of the leaves. But matter also leaves the plant daily 

 as carbon dioxide (in small quantities), water, and oxygen, both by 

 diffusion through the epidermis and by transpiration through the 

 stomata. Strictly speaking, the term outgo should be restricted 

 to the output of matter which has at some time actually formed 

 a part of the living protoplasm, and this does not apply to the 

 oxygen, which is simply given off in the manufacture of starch, 

 or to the bulk of the water of transpiration which passes straight 

 through the plant without undergoing any chemical change. En- 

 ergy likewise leaves the plant continuously both as heat and in 

 the doing of mechanical work, both of which are involved in 

 every vital act. 



Respiration. It has been remarked that in the light (i.e., when 

 manufacturing starch) Pteris takes in carbon dioxide and gives off 

 free oxygen. But if the plant be deprived of light, as at night, the 

 reverse is true, and the plant takes in a small amount of oxygen 

 and gives off a corresponding amount of carbon dioxide. 

 This latter process is the true breathing or respiration of the 

 plant, and it must not be confounded with the taking in of 

 carbon dioxide and giving off of oxygen. Respiration goes on in 

 the light also, probably with greater energy than in darkness, but 

 it is then partially obscured by the other and more conspicuous 

 process. 



We have seen that energy is set free in living matter by a 

 decomposition of its own substance which is really a process of 

 oxidation or combustion, in which free oxygen plays an impor- 

 tant part (p. 34, Chap. III.) ; hence the absorption of free oxy- 

 gen in respiration. Among the products of the combustion, 



