Physiology of Plants

PHYSIOLOGY OF PLANTS The physiology of plants deals with the functions of the living plant as morphology and anatomy deal with form and structure.

Physiology aims to explain the functions of an organism in terms of physical and chemical processes. Physiology thus consid ers the living organism solely from the mechanistic standpoint, i.e., from the standpoint that the organism is a machine albeit a very complex one. In this attempt to explain the organism in the terms of a machine the physiologist is supported by the fact that the organism appears to obey so-called physical laws in much the same way as does dead material. The living organism for example obeys the law of the conservation of energy, the amount of energy given out by the organism being equal to the amount taken in in food; again the plant or animal conforms also to the second law of thermodynamics which relates to the use of free energy.

Characteristics of Living Organisms.

Although it is im possible to give an exact definition of a living organism some of its characteristics may be pointed out. Whether one is dealing with a plant or animal of a single cell, or of a large organism such as an elephant or a tall tree, it is evident that it is a machine constructed to carry on certain processes, such as the absorption or building up of food; the process of respiration (see below) and the processes of growth and reproduction. One great difference between the living machine and any other machine is that the living machine is infinitely more complex than any man-made machine. Then again the processes going on in the organism are not independent of one another but are to a high degree, what is called, integrated. All the processes are interlinked or related, the finished material of one process being often the raw material of the next, and all being so closely associated as to lead to move ment or growth or reproduction or some other activity of the organism. Another marked characteristic of the living machine which distinguishes it from all other machines is its power, to put it colloquially, to build itself up as it goes along. In other words

the growing organism is a functioning unit which adds to itself and increases the size of the machine by its own functional activity. Another characteristic of the plant or animal is its con stant reaction to the environment ,and its constant adaptation to changes in the environment. It is true that a carefully dried seed kept at a low temperature may appear to be quite inert, though still alive as is shown by its power of germination under suitable conditions ; yet all active organisms show a constant interchange with their environment. Even in the case of the seed it is prob able that some interchange, such as respiration, is still going on though at such a slow rate that our present methods of analysis are too coarse to recognize it. Another characteristic of the organism is that any apparent steady state or equilibrium is not a static but a dynamic one. If the normal organism appears to be in an inactive state it is simply that the constructive processes just balance the destructive processes, not that the processes are at a standstill. The living organism is like a battery which is constantly running down and requires constant recharging, though the recharging goes on at the same time as the discharging. The majority of cells require oxygen for their mere continued exist ence even in a resting condition and additional oxygen when in a state of activity. This need of oxygen even during quiescence indi cates that a quiescent state of the cell is only an apparent quies cence, a state of dynamic not static equilibrium. Again the giving out of heat by all living organisms even when apparently inactive indicates the dynamic equilibrium of chemical processes going on in the cells. These exceedingly intricate processes continue unceasingly throughout the life-period of every cell in the simplest as well as in the most complex organisms.