Let us make an in-depth study of the growth in plants. After reading this article you will learn about 1. Definition, Regions of Growth and Growth Correlations 2. Kinetics of Growth – The Course of Growth (Grand Period of Growth) or Sigmoid Curve (Grand Period Curve) and 3. Measurement of Growth.
Definition, Regions of Growth and Growth Correlations:
Growth in a plant is the outcome of cell division, enlargement of the new cells and their differentiation into different types of tissues. These processes of growth are accompanied by (i) a permanent change in size (usually an increase in length or volume) and (ii) an increase in the dry weight of the growing parts.
In plants growth is confined only to meristems. The extreme apices of root and shoot, for instance are occupied by primary meristems while in their older parts, secondary meristems (i.e., cambia) give rise to additional vascular tissues and to protective layers of cork cells.
The activity of each meristem influences the activity of the other meristems especially those near to it, giving rise to what is called as growth correlations. For instance, while the main apical shoot meristem is active it retards the activity of more recently initiated lateral bud meristems, a phenomenon usually referred to as ‘apical dominance’.
Kinetics of Growth – The Course of Growth (Grand Period of Growth) or Sigmoid Curve (Grand Period Curve):
Usually under favourable conditions there is a characteristic course of increase in the plant in any of its growing parts. Growth is slow at first (Lag Phase), then gains speed (Log Phase) and eventually slows down (Decreasing Growth Rate) to come to a halt (Steady State). The total time during which this course of growth takes place is called as the Grand Period of Growth. If this growth rate is plotted against time, a S shaped curve is obtained which is called as Sigmoid Curve or Grand Period Curve (Fig. 17.1).
The sigmoid curve represents the integrated sum of the curves for each growing organ and cell and presents the changing size of these parts. Similarly, when dry weight is measured as an index of growth before maturity, the curve takes the well known sigmoid form. Environmental conditions may alter growth rates but not the sigmoid form of the growth curve.
In unicellular organisms such as Chlamydomonas or bacteria, growth is assessed by a count of number of cells per millilitre at increasing times after the cells are placed in a fresh nutrient medium and under environmental conditions (light, temp, etc.) suitable for optimal growth. Here also, there is initial lag period during which cells activate their biochemical machinery for rapid growth by synthesizing necessary enzymes. This is followed by a time period during which there is exponential increase in cell number which is called as log period.
This period of rapid growth does not continue indefinitely and due to depleted nutrient supply, accumulation of toxic products and other limiting factors ultimately leads to decreasing cell number until the population of cells reaches a steady state in which the number of cells remains constant (stationary) or even declines. If number of cells per millilitre is plotted against time (hours), again a sigmoid curve is obtained as described earlier.
The ratio of the change in cell number (dn) over the time interval (dt) is called as absolute growth rate (AGR):
AGR = dn/dt
The AGR ‘when divided by total number of cells present in the medium, gives relative growth rate (RGR):
RGR = AGR/n
AGR and RGR are useful because they help to describe dynamics of cell growth in culture.
Measurement of Growth:
Growth in plant can be measured in terms of either:
(i) an increase in length or girth as in case of stem and root, or (ii) an increase in weight, or (iii) an increase in volume or area as in case of fruits and leaves respectively.
The following methods are usually employed for the measurement of growth in length:
(1) Direct Method:
This is the most simple rather crude method of the measurement of growth in which the length of the growing part is measured just by the help of a scale after intervals.
(2) Horizontal Microscope:
In this method, a point is marked near the stem or the root tip and is focused by horizontal microscope which slides over a graduated vertical stand (Fig. 17.2). After some time the same point is again focussed either by raising (in case of stem tip) or lowering (in case of root tip) the horizontal microscope. The difference of the initial and final readings on the graduated vertical stand measures the growth of stem or the root tip in length.
(3) Arc Auxanometer:
The increase in length of the stem tip can easily be measured by an arc auxanometer which consists of a small pulley to the axil of which is attached a long pointer sliding over a graduated arc as shown in the Fig. 17.3. A thread, one end of which is tied to the stem tip and the other end to a weight passes over the pulley tightly.
As soon as the stem tip increases in length, the pulley moves and the pointer slides over the graduated arc. The reading is taken. Actual increase in length of the stem is then calculated by knowing the length of the pointer and the diameter of the pulley. If the diameter of the pulley is 4″ and the length of the pointer 20″, growth is magnified ten times on the graduated arc.
(4) Pffefer’s Auxanometer:
It consists of a compound pulley with a small and a large wheel both having the same axil. A thread, one end of which is tied to the stem tip and the other end to a weight passes over the smaller wheel tightly. Another thread whose both ends are tied with weights tightly passes over the larger wheel. Near one end of this thread is attached a pointer which remains in touch with a rotating cylindrical drum covered by a smoked paper as shown in the Fig. 17.4.
As soon as the stem tip increases in length, the wheels of the pulley move so that the pointer also moves downward and traces a special white marking on the smoked paper which gives an idea of the growth in the stem tip. Actual increase in length of the stem can be calculated by knowing the radii of larger and smaller wheels and the rate of rotation of the drum.