Quantitative alteration of External Conditions 231 
factor is found to be a reduction in the supply of nutritive salts and 
the exposure of the plants to prolonged illumination or, better still, 
an increase in the intensity of the light, the efficiency of illumination 
depending on the consequent formation of organic substances such as 
carbohydrates. 
The quantitative alterations 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 distri- 
bution 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 pro- 
duced 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; 
eg. the growing apices of buds which serve as cuttings for repro- 
ductive purposes, or buds on tubers, runners, rhizomes, etc. A grow- 
ing 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, de- 
pendent simply on the specific structure, which compels a plant to 
produce its flowers after a definite period of vegetative growth}. 
+ Klebs, Willkiirliche Entwickelungsdnderungen, Jena 1903; see also ‘Probleme der 
Entwickelung, 1, 1.” Biol. Centralbl. 1904. 
