Quantitative alteration of External Conditions 231 



factor is foimd 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; 

 e.g. 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 dilferent 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. Tlie 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 \ 



1 Klebs, Willkiirliche Entwichelungsamlerungen, Jena 1903 ; see also " Probleme der 

 Entwickelung, i. ii." Biol. Centralbl. 1901. 



