General Metabolism of the Plant

Periodicity of Growth.

Organs of limited growth usually show what is called a "grand period of growth," the rate of growth being at first slow then gradually increasing to a maximum and then slowing down again to zero. This may be illustrated by a zone on the main root of a bean seedling which in successive days showed the following increases, the results being expressed in mil limetres, 1.8, 3.7, 14.5, 7.o, 0.0. If we express the total length of the root section graphically we have an S shaped curve; the organ or plant body starts as a small structure, grows at first slowly and then with increasing rapidity, and then the rate of growth begins to slow down and finally stops.

is negligible but the one-sided action of gravity is abolished as every side of the organ in turn faces downwards; the effect of gravity is not annulled but the curving (tropistic) effect is abol ished. It is evident that the curvature results from the differ ence in the elongation of the two sides.

Geotropic Curvature

is not confined to plant organs which are still in active growth. It may occur in organs which have normally ceased to grow as in the case of the stems of grasses.

If a short piece of wheat or barley stem is placed in a horizontal position, the base of the node, to the growth of which the elonga tion of the node is mainly due, will start into growth again as a result of the stimulus of gravity and the stem turns upward. That such curvature is not a simple weight effect on the plant is shown by the upward curvature of the stem, and by the fact that the root often will grow downwards into mercury although in such a dense fluid the root tends to be forced upwards.

Reaction and Presentation Time.

As we are dealing with a stimulus we should expect the effect to be related to the duration, intensity and direction of the force; this we find to be the case.

When a stem or root is laid horizontal the time taken for the organ to show a visible curvature is i o to 20 minutes, according to the sensitiveness of the object and the external conditions, such as the temperature. This period is known as the reaction time. It is found that this length of time is not necessary to produce a curvature. If after a much shorter time the plant is removed from the horizontal position and placed on a klinostat, the curvature follows later. The minimum time to which the plant must be exposed to the gravitational stimulus in order to show later a curvature is known as the presentation time. This is much shorter than the reaction time and varies from as short a period as 2 min. for the inflorescence stalk of the plantain and the

hypocotyl of the sunflower, to 25 minutes for some other plants. There is naturally a marked difference in the presentation times exhibited by individuals of the same species. In the oat shoot, for example, the mean time was 32 minutes, but some reacted in less than 14 minutes, while others required more than 49 minutes.

It is found that the stimulation applied during the presentation time need not be given all at once. A series of short exposures, each less than the presentation time, may by summation become effective. If the pauses between stimulation are only five times as large as the periods of stimulation the presentation time is not lengthened by the intermittent nature of the treatment. Thus, if the normal presentation time of a shoot is i o minutes, it will react if placed horizontal for five successive 2 minute periods, with a period of 10 minutes in the vertical position in between. If, however, the period between one stimulation and the next is 20 minutes, then more than 5 of the two-minute periods will be required. The effect of the stimulation thus slowly weakens dur ing the pauses.

The presentation time and reaction time are naturally affected by the intensity of the applied stimulus. We cannot alter the effect of gravity, but we can use instead a centrifugal force which can be increased at will and to which, as has been shown above, plants respond as to gravity. The table below gives the intensity of the centrifugal force measured in terms of gravity (G), the presenta tion time, and the products of the time by the intensity.

Correlation of Growth.

One of the most striking phe nomena in the growth of the plant body is the correlation of growth. The thickness of the trunk is related to the size of the branches which it bears, and the size of the root-system is cor related with that of the shoot-system. A good example of cor relation is seen in the bending up in a side-shoot to replace the loss of the top of the main stem in the tree trunk of a conifer. It has been known for some time that in the animal body hormones (chemical messengers) are produced by certain glands and that these hormones circulating about the body may control growth and other processes. It has of recent years been demonstrated that the plant possesses growth-promoting substances which can dif fuse through the plant and so control growth at some distance from the point of production of the substances.