terations of external conditions may be spoken of as
releasing stimuli. They produce, in the complex equilibrium of the cell,
quantitative modifications in the arrangement and distribution of mass,
by means of which other chemical processes are at once set in motion,
and finally a new condition of equilibrium is attained. But the
commonly expressed view that the environment can as a rule act only as
a releasing agent is incorrect, because it overlooks an essential point.
The power of a cell to receive stimuli is only acquired as the result
of previous nutrition, which has produced a definite condition of
concentration of different substances. Quantities are in this case
the determining factors. The distribution of quantities is especially
important in the sexual reproduction of algae, for which a vigorous
production of the materials formed during carbon-assimilation appears to
be essential.

In the Flowering plants, on the other hand, for reasons already
mentioned, the whole problem is more complicated. Investigations on
changes in the course of development of fertilised eggs have hitherto
been unsuccessful; the difficulty of influencing egg-cells deeply
immersed in tissue constitutes a serious obstacle. Other parts of plants
are, however, convenient objects of experiment; e.g. the growing apices
of buds which serve as cuttings for reproductive purposes, or buds on
tubers, runners, rhizomes, etc. A growing apex consists of cells capable
of division in which, as in egg-cells, a complete series of latent
possibilities of development is embodied. Which of these possibilities
becomes effective depends upon the action of the outer world transmitted
by organs concerned with nutrition.

Of the different stages which a flowering plant passes through in the
course of its development we will deal only with one in order to show
that, in spite of its great complexity, the problem is, in essentials,
equally open to attack in the higher plants and in the simplest
organisms. The most important stage in the life of a flowering plant
is the transition from purely vegetative growth to sexual
reproduction--that is, the production of flowers. In certain cases it
can be demonstrated that there is no internal cause, dependent simply
on the specific structure, which compels a plant to produce its flowers
after a definite period of vegetative growth. (Klebs, "Willkurliche
Entwickelungsanderungen", Jena 1903; see also "Probleme der
Entwickelu