INFLUENCE OF EXTERNAL CONDITIONS ON GROWTH 29 1 



There are many plants whose flowers open normally in darkness, so long as 

 the rest of the plant is exposed to light. In some cases the form of the flowers 

 produced is dependent upon light conditions. Thus, Vochting 1 found that the 

 formation of cleistogamous flowers (which are self-pollinated and never open) is 

 markedly influenced by external conditions, especially by light. The plants of 

 Vochting's experiment were placed on the inner side of, and at various dis- 

 tances from, a northeast window, so that they received light of various inten- 

 sities. With some plants the effect of being placed farther from the source of 

 light produced only a decrease in the number and size of the flowers, but the 

 flowers opened in all cases. In the case of plants with a tendency toward 

 cleistogamy, however, the number of cleistogamous flowers produced increased 

 as the plants were farther from the window. With such plants it is possible to 

 obtain either ordinary or cleistogamous flowers at will, by controlling the light 

 intensity during the flowering period. 



The flowers of many plants are open only by day and are closed at night 2 

 (see Fig. 130, p. 279), while those of some other plants are open only at night 

 and are closed by day. These periodic movements of petals and sepals are 

 frequently dependent upon light variation, and, as is shown by measurements, 

 they are directly due to unequal growth on the two sides of the organ. When 

 growth of the outer or lower regions of the petals is more rapid, the flower 

 closes, and opening occurs when growth is more rapid on the inner or upper 

 side. Such movements of floral parts may also be brought about by tempera- 

 ture changes, to which many flowers are especially sensitive in this way; thus, 

 a temperature change of 5°C. is sufficient to produce complete closing or 

 opening of Crocus flowers. 



Light also exerts an influence upon the development of lower plants, such 

 as fungi. 3 Pilobolus, for example, develops normally in. weak light but pro- 

 duces very long sporangiophores in darkness, where, also, the spores fail to ma- 

 ture. Light is injurious to colorless bacteria, which are killed by direct sunlight 

 and hindered in their growth by diffuse light. This is shown very beautifullv 

 by H. Buchner's experiment. He pasted black paper letters on the bottom of a. 

 Petri dish containing a freshly prepared plate culture of typhus bacteria iru 

 nutrient agar, and then exposed the dish, bottom upward, to direct sunshine 

 for one and a half hours. The dish was then placed in darkness for twenty-four 

 hours, after which, when the black paper was removed, the forms of the letters 

 could be plainly seen in the agar plate, because of the numerous white colonies 

 that had developed, exclusively where the bacteria had been protected from the 



ness of photographic paper does not vary in the same way, with the wave-length of impinging 

 radiation, as does the effectiveness of the radiation to favor plant growth and development. 

 The problem is an exceedingly complex one, for which none but very general methods may 

 even be suggested at present, but progress may be best furthered by a frank appreciation of the 

 logical requirements. — Ed. 



1 Vochting, Hermann, Ueber den Einfluss des Lichtes auf die Gestaltung und Anlage der Bltithen. 

 Jahrb. wiss. Bot. 25: 149-208. 1893. 



- Pfeffer, W., Physiologische Untersuchungen. Leipzig, 1873. 



3 Brefeld, [O.], Ueber die Bedeutung des Lichtes fur die Entwickelung der Pilze. Bot. Zeitg. 35: 386 

 401-408. 1877. 



