An auxanometer (Gr. auxain, "to grow" + metron, "measure") is an apparatus for measuring increase or rate of growth in plants.

In case of an arc-auxanometer (see picture), there is a wire fixed with the plant apex on one end and a dead-weight on the other.

In growth curve, maximum growth occurs in exponential phase of sigmoid curve. During this periods cells divide mitotically to increase length and girth of the plant.

Irreversible increase in mass or volume of plant part is called apparent growth. Apparent growth is permanent and cannot be reversed back as it occurs due to increase in cell number.

S- Shaped growth curve (Sigmoid growth curve) A pattern of growth in which, in a new environment, the population density of an organism increases slowly initially, in a positive acceleration phase; then increases rapidly approaching an exponential growth rate; but then declines in a negative acceleration phase until at zero growth rate the population stabilizes. This slowing of the rate of growth reflects increasing environmental resistance which becomes proportionately more important at higher population densities. This type of population growth is termed ‘density dependent’. [Add image here]

An inflection point is a point on a curve at which the sign of the curvature (i.e., the concavity) changes. Inflection points may be stationary points, but are not local maxima or local minima.

For example, for the curve plotted given, the red point is an inflection point.

The tissues that have ability of cell division located in specific area are called meristem. They are located at stem and root tips, nodes of stem and intercalary region.

Growth is defined as a permanent or irreversible increase in dry weight, size, mass or volume of a cell, organ or organism. Growth in living beings is intrinsic or internal.

Secondary growth is the result of the activity of the two lateral meristems, the cork cambium and vascular cambium. Arising from lateral meristems, secondary growth increases the girth of the plant root or stem, rather than its length.

Arithmetic growth is represented by where Lt is final growth, Lo is initial size and rt is the rate of growth and time respectively.

Testing of biological activity like growth response of a substance by employing a living material like plant or plant part is called bioassay. It is used to know the effect of substance on plant growth.