302 METAMORPHOSIS 



of plant life, and indeed of all organisms, since it provides the energy whereby 

 the green plant builds up organic material, on which in turn all organized life 

 on the earth depends. We are not, however, concerned here with this aspect 

 of the case. If we provide for the supply of a sufficient quantity of nutrient 

 materials, then the great majority of plants and plant parts can grow without 

 light. Light is not generally essential to that condition of capacity for growth 

 which we may term phototonus, in analogy with thermotonus ; in other words, 

 withdrawal of light does not, as a rule, induce a darkness-rigor in the plant. 

 In many cases which we may describe as darkness-rigor, we are dealing 

 in reality with phenomena which are only indirectly dependent on the absence 

 of light (JosT, 1895). Only in a few cases can it be shown that light is an essen- 

 tial formal condition of growth. Thus it has been established that many seeds 

 germinate badly or not at all if they be kept constantly in the dark ; Viscum 

 album (WiESNER, 1894), Veronica peregrina (Heinricher, 1899), and Nicotiana 

 (Raciborski, 1900) may be cited as conspicuous examples of this. This 

 phenomenon is especially common in the spores of ferns and mosses (Borodin, 

 1868 ; Leitgeb, 1876). That we have not in this instance to deal with an 

 assimilatory action of light and the renewal of the necessary constructive 

 material, but with a specific stimulatory action, is shown by the fact that if 

 tobacco seeds soaked in water be exposed to light for an hour they can germinate 

 in darkness, further, that germination of the spores above mentioned may take 

 place in air from which carbon-dioxide has been excluded, and, finally, that 

 light may be replaced by other stimuli, such as a temperature of 32° C. in the 

 case of fern-spores, and a sugar solution in the case of moss-spores (Heald, 

 1898 ; Goebel, 1896). 



In such cases as these, which must be considered as exceptions to the rule, 

 we may speak of a minimum intensity of light essential to development. There 

 is a definite m,aximttm, however, affecting the generality of plants, which, when 

 exceeded, first retards growth, and, finally, causes death. The position of the 

 maximum is again specifically very varied. It lies very low in shade-loving 

 plants such as we find abundantly in woods, or, more especially, in the sea. 

 Such plants are killed by direct sunlight. The same is the case with many 

 Bacteria, which are killed by a brief exposure to direct sunlight, or even diffuse 

 light. The light maximum is much lower still for many subterranean organs than 

 it is for shade-loving plants. Thus it is well known that the tubers of the potato 

 bud out readily in the dark, while such development is retarded in diffuse 

 daylight. Darkening has a similarly favourable effect on many, but not all 

 roots, as we shall see later on. Plants which naturally grow in sunny regions 

 are those best adapted to the highest light intensity, and the maximum in their 

 case is reached only when the sunlight is concentrated by means of a lens. The 

 different organs of the plant are not all equally sensitive ; thus the chloroplasts 

 are much more easily injured than the rest of the protoplasm. 



During the course of development of individual organs the position of 

 the light maximum varies often very considerably. It is only while in the young 

 state that the shoots of the potato are strongly retarded in their growth by light. 

 We meet with very remarkable relationships in the Cactaceae, whose shoots in the 

 long run lose their power of growth as a result of illumination. When put in the 

 dark growth recommences and continues for a time after re-illumination. The 

 growing point may be stimulated to further growth if placed in the dark. Many 

 aquatics behave in a similar manner, e. g. Elodea, Ceratophyllum, Myriophyl- 

 lum, for MoBius (1895) found that the full-grown internodes of such plants 

 started growing once more when placed in darkness. On the other hand, light 

 exercises a retarding influence on the embryonic growth at the growing point 

 in the Cactaceae and on the growth in length of internodes in the aquatics 

 mentioned. 



