SUPPLEMENT gi 



the leaf on sunlight may, on more careful reflection, be explained otherwise. 

 It is well known (SACHS, 1863) that the leaves of many Leguminosae in the 

 dark lose in a short time their special powers of movement in response to 

 stimulus (Lect. XXXIX) ; they suffer from darkness-rigour and die off. Other 

 leaves also which grow in light die rapidly when placed in the dark. Doubtless 

 we have to deal in this case with a secondary destructive effect due to with- 

 drawal of light, for if leaves are allowed to develop in the dark from their young 

 state, and if suitable nutriment be provided, they reach their normal size and 

 form and remain alive for a long time (JosT, 1895). It is, of course, obvious 

 that an auto trophic plant cannot exist altogether without light, but its death 

 is due to an absence of carbohydrate, for the formation of which light is essential. 



Setting aside all the phenomena already spoken of, there remain but few 

 facts which go to prove that light is an essential factor in development. Thus 

 it has been proved that many seeds do not germinate, or do so badly, if they 

 are kept perpetually in the dark ; Viscum album (WiESNER, 1894), Veronica 

 peregrina (HEINRICHER, 1899), and Nicotiana (RACIBORSKI, 1900) may be cited 

 as conspicuous examples. That we have to deal in these cases not with an 

 assimilatory effect of light, i. e. a renewal of the necessary food-stuff, but with 

 a specific stimulatory effect, is proved by this fact amongst others, that in 

 tobacco, for example, illumination for one hour of seeds soaked in water is 

 sufficient to induce germination even in darkness. 



In addition to the seeds above mentioned, there are also the spores of 

 mosses and ferns which, according to BORODIN (1868), and LEITGEB (1876), 

 germinate only in light. It has been shown, however, that a high temperature 

 or material influences may be substituted for light in this case (GOEBEL, 1896 ; 

 HEALD, 1898 ; TREBOUX, 1903). According to LAAGE (1907) the facts of the 

 case may be expressed by saying that very many spores are able to germinate 

 in darkness, but that they then make greater demands on the substratum ; the 

 accession of light enables them to overcome any unsuitable combinations in 

 the substratum. 



From these statements, it follows that a certain minimum intensity of 

 light is a general condition of growth, just as is a minimum temperature, but 

 that, all the same, any definite minimal intensity of light is not essential for 

 the ' normal ' elaboration of many plant organs. There is, however, a general 

 maximum of light intensity which must not be exceeded, otherwise growth is 

 retarded and the plant ultimately dies. The position of this maximum is, 

 again, very different according to the species concerned. It lies low in shade- 

 loving plants, such as inhabit woods, and such as are found specially abundantly 

 in the sea. In direct sunlight such plants die. Many Bacteria also exhibit 

 a similar sensitivity to light, for after a short exposure to its influence even 

 when nearly diffuse they are killed. The light-maximum lies far lower when 

 subterranean organs are in question ; it is known, for instance, that buds on 

 potato tubers readily germinate in the dark, though daylight even if 

 diffuse retards the growth. A similar favourable influence of darkening has 

 been established for many, though not all, roots, but these we shall discuss later 

 on. Plants which are able to endure the highest light intensities, and which 

 thrive under natural conditions in many situations, reach their maximum light 

 intensity only when the sunlight is concentrated artificially by the aid of lenses. 

 Not all organs of a plant are equally sensitive ; the chloroplasts are often more 

 rapidly injured than the protoplasm (PRINGSHEIM, 1879), and in this way may 

 be explained a whole series of adaptations in the plant, aimed at the protection 

 of the chlorophyll from excessive illumination (WiESNER, 1876, 1894). 



Read P. 302, 11. 43-55. 



303, 11. 1-4, for Just as ... optimum read Light intensities lying between 



