Ingenhousz in 1779 discovered that plants
incessantly give out carbonic acid gas, but that the green leaves and
shoots only exhale oxygen in sunlight or clear daylight, thereby
indicating the distinction between assimilation of carbonic acid gas
(photosynthesis) and respiration. N.T. de Saussure (1767-1845) gave
precision to the science of plant-nutrition by use of quantitative
methods. The subjects of plant nutrition and respiration were further
studied by R.J.H. Dutrochet towards the middle of the century, and
Liebig's application of chemistry to agriculture and physiology put
beyond question the parts played by the atmosphere and the soil in the
nutrition of plants.

The phenomena of movements of the organs of plants attracted the
attention of John Ray (1693), who ascribed the movements of the leaf of
Mimosa and others to alteration in temperature. Linnaeus also studied
the periodical movements of flowers and leaves, and referred to the
assumption of the night-position as the sleep-movement. Early in the
19th century Andrew Knight showed by experiment that the vertical growth
of stems and roots is due to the influence of gravitation, and made
other observations on the relation between the position assumed by plant
organs and external directive forces, and later Dutrochet, H. von Mohl
and others contributed to the advance of this phase of plant physiology.
Darwin's experiments in reference to the movements of climbing and
twining plants, and of leaves in insectivorous plants, have opened up a
wide field of inquiry as to the relation between plants and the various
external factors, which has attracted numerous workers. By the work of
Julius Sachs and his pupils plant physiology was established on a
scientific basis, and became an important part of the study of plants,
for the development of which reference may be made to the article
PLANTS: _Physiology_. The study of form and development has advanced
under the name "morphology," with the progress of which are associated
the names of K. Goebel, E. Strasburger, A. de Bary and others, while
more recently, as cytology (q.v.), the intimate study of the cell and
its contents has attracted considerable attention.

The department of geographical botany made rapid advance by means of the
various scientific expeditions which have been sent to all quarters of
the globe, as well as by individual effort (see PLANTS: _Distribution_)
since the time of A. von Humboldt. The question