ELEMENTARY MORPHOLOGY AND PHYSIOLOGY. I I 



The expenditure of energy on the part of plants involves, as 

 previously mentioned, a constant decomposition of protoplasm. 

 This has been termed Katabolism. Such products of breaking- 

 down may be passed out of the organism or excreted. Carbon 

 dioxide is such a icaste 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, i.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. i). 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. i ). 

 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. It must be borne in mind that 



