ELEMENTARY MORPHOLOGY AND PHYSIOLOGY. 1 
The expenditure of energy on the part of plants involves, as 
previously mentioned, a constant decomposition of cell-contents. 
This has been termed Katabolism. Such products of breaking- 
down may be passed out of the organism or excreted. Carbon 
dioxide is such a waste product, and the passage out of this (and 
water), with concomitant passage in of oxygen, is known as 
Respiration. The oxygen effects the decomposition, which is a 
process of oxidation. It is most important to remember that all 
organisms, plant and animal, with very few exceptions, respire or 
breathe, and in the same way, 7.e., by taking in oxygen and 
giving out carbon dioxide. It is popularly, but very erroneously, 
stated that “plants breathe in carbon dioxide and breathe out 
oxygen.” Green plants do, in the presence of light, give off 
oxygen, as seen above, but this is simply a part of their food 
which they do not utilize. The amount of oxygen is so large 
as to disguise the fact that carbon dioxide is also being evolved, 
though in much smaller quantities. The true state of things 
becomes apparent at night, when, since the chlorophyll is not at 
work, the evolution of carbon dioxide is not masked. This is 
why plants help to vitiate the air at night, and are therefore best 
excluded from a sleeping-room. 
If a plant receives and assimilates abundant food, it will grow 
to a certain extent, and also reproduce or give rise to new indi- 
viduals. Reproduction, in a simple unicellular form, such as we 
are considering, takes place most simply by division or fission 
into two equal parts (fig. 1). The nucleus divides into two—a 
cellulose partition is formed across the cell, and the two halves 
gradually round off and separate. 
Our example may also exhibit movements, and such Motility is 
most commonly seen in the form of currents in the protoplasm, 
which are rendered evident by the presence of granules (fig. 1). 
These are swept along from place to place. 
The last physiological heading is that of Irritability and Spon- 
taneity. In other words, the organism is sensitive to agents 
or stimuli (mechanical, chemical, thermal, &c.), which act upon 
it from without or within. Irritability means sensitiveness to 
external stimuli. The protoplasmic currents, for example, men- 
tioned in the last paragraph, can, to a certain extent, be altered 
in rapidity by raising or lowering the temperature of the external 
medium. On the other hand, spontaneity is sensitiveness to 
internal stimuli. Movements of protoplasm, to take the same 
instance, are often so constant that it is scarcely possible to sup- 
pose them entirely the direct result of external influences. They 
must be regarded as spontaneous, or the result of internal stimuli, 
as chemical change, &c., &c. It must be borne in mind that 
