308 METAMORPHOSIS 



Vienna to one twenty-eighth at Drontheim, and to one-fifth at Tromso ; the 

 minimum of the absolute photic ration in Betula nana is 0-338 at Christiania, 

 0-386 at Tromso, and 0-750 at Spitzbergen. 



WIESNER draws the following conclusion from his observations : ' Just as 

 the plant requires a certain amount of heat for the proper performance of its func- 

 tions, so also it needs a certain definite amount of light.' It has not, however, 

 been proved that the plant requires a certain amount of heat, we only know that 

 it needs a certain degree of temperature. Unless this be provided it is not only 

 impossible for it to thrive but all growth is impossible. Branches of the above- 

 mentioned plants can also grow without any light, and so light cannot be con- 

 sidered as an absolutely necessary condition of life in the same way as tempe- 

 rature (p. 301). 



The formative influences of light hitherto discussed refer to disturbances in 

 the normal correlations of organs, and express themselves in increase on one side 

 and decrease on the other ; when total exclusion of light is effected and the plant be- 

 comes etiolated these phenomena are most clearly marked. When, however, we 

 speak of an etiolated plant yet another modification of the normal plant occurs to 

 us, viz. alteration in colour. Etiolated plants have white stems and yellow leaves, 

 since chlorophyll is not usually formed in darkness. We may with good reason, 

 however, distinguish between this colour change and real etiolation, by which we 

 understand only 'excessive elongation' and 'reduction,' since etiolation may 

 take place in cases where chlorophyll is present. Later on we shall become ac- 

 quainted with factors, other than darkness, which induce excessive elongation 

 without injuring the chlorophyll. Moreover, there are quite a number of plants 

 in which the formation of chlorophyll is independent of light and in which all 

 the same a marked etiolation takes place in darkness (ScniMPER, 1885). This 

 appears to be the case generally with Algae and mosses, while Pteridophyta 

 behave variously. The Equisetaceae, like Phanerogams, form no chlorophyll 

 in the dark, whilst the Filicinae do. The Gymnosperms are especially interest- 

 ing ; while the adult plant can form no chlorophyll in the dark, the seedlings of 

 Coniferae and of Ephedra have that power (SACHS, 1862 and 1864; BURGERSTEIN, 

 1900). In cases where the formation of a yellow-colouring matter only takes 

 place in chloroplasts in the dark, it is often quite sufficient to expose leaves which 

 are not too old for a short time to sunlight in order to obtain the green coloration. 

 If we again place the plant in darkness, after the light has acted for a time on 

 the etiolated leaf, but before the light has produced any visible effect, the 

 greening takes place in the dark as an after-effect. A very low light intensity 

 is sufficient for the purpose, greater intensities indeed tend to destroy the 

 chlorophyll. 



All vegetable colouring matters do not behave like chlorophyll. SACHS 

 (1863) showed that many plants formed normally coloured flowers in darkness 

 (e. g. tulip, crocus, cucumber, &c.) ; in other cases, however, these floral colours 

 are formed only in light (ASKENASY, 1876). Similarly the formation of the red- 

 colouring matter, often found dissolved in cell-sap, depends entirely on the 

 presence of light (OVERTON). In all these examples we are dealing with effects 

 of light which may influence very greatly the general appearance of the plant, 

 but which are perhaps quite simple phenomena, to be explained by a sufficiently 

 thorough knowledge of the, chemical composition of the colouring matter in 

 question. We have still, however, other formative influences of light to con- 

 sider which present more difficult problems for solution. 



There are many plants of varied relationship which exhibit a different 

 form when young from that exhibited by them when more fully developed. 

 In many of these the juvenile form is produced by being exposed to light of less 

 intensity than in older stages. If the intensity of light remains low, the 

 later form is suppressed ; if the intensity of light diminishes after the adult 



