306 METAMORPHOSIS 



Etiolation is not limited to Monocotyledons and Dicotyledons ; it has been 

 observed also in Gymnosperms, ferns, mosses, Algae, and Fungi. As to some 

 of these cases we shall have something to say. later on at present a few exam- 

 ples only, taken from the Fungi, need be referred to (compare PFEFFER, Phys. II, 

 102). The influence of darkness is very marked on certain species of Coprinus, 

 where a vigorous elongation of the stipe and a diminution of the pileus occurs. 

 In extreme cases, in some species, the pileus is entirely suppressed (e. g. C. ster- 

 corarius), a result which can no longer be considered as a case of etiolation. 

 An excessive elongation of the sporangiophore has been observed in many 

 Mucorineae (Pilobolus), and the stalk of the perithecium of Sphaeria velata has 

 been known in darkness to elongate to five times its usual length. 



The instances of etiolated Fungi last cited are of especial interest when we 

 attempt to answer the question as to the causes of etiolation. In these cases 

 the accessory action of light (in carbon assimilation) is quite excluded from con- 

 sideration. We can certainly prove that darkening the higher green plant does 

 not induce etiolation by stopping carbon-dioxide assimilation. If we cultivate 

 an autotrophic plant in light, but in an atmosphere without carbon-dioxide, we 

 exclude all carbon-dioxide assimilation, yet no etiolation is to be observed. 



The varied behaviour of the different organs of plants, as well as that of 

 different species, shows also that in etiolation we have to deal with a stimulatory 

 action of light, which under different conditions leads to the most diverse 

 results. In the first place we have an alteration in the normal correlation of 

 growth between organs ; the great development of the internodes hinders, in 

 Dicotyledons, the usual surface-growth of the leaf-blade. Similarly we can 

 induce the formation in darkness, in Phaseolus and Mimosa, of leaves of about 

 normal size ( JOST, 1895) if, by removing all lateral buds before their elongation, 

 the main shoot is saved from competition with them. PALLADIN (1890) has 

 obtained the same result by retarding growth in the internodes by appropriate 

 means. It is not, however, known in detail how illumination or darkening 

 operates on the growth of cells, and the numerous experiments which have 

 been made lay stress for the most part on one possible factor only, such as turgor, 

 elasticity of the cell-wall, &c., thus implying that the problem is simpler than it 

 really is. It must be remembered amongst other things that etiolation is not 

 a simple consequent of light activity, but is rather a complex result of several 

 secondary influences, especially the retardation of transpiration (compare p. 319). 



We know much more about the biological significance of etiolation than 

 we do about its causes (GODLEWSKI, 1889; DARWIN, 1896). Looking upon 

 superelongation of certain organs as the most usual characteristic of etiolation 

 we may regard it as an adaptation on the part of the plant to escape from dark- 

 ness. From this point of view it is immaterial whether the internodes or the 

 petioles elongate ; the chief point is that the special organs which require illu- 

 mination should be lifted out of the darkened area. The leaves need not enlarge 

 if they are not to exercise their functions. Further investigation is required 

 to show how far the superelongation of the fruit-bearing parts of Fungi or 

 mosses is of service. In the case of experiments carried out in the dark etiolation 

 is certainly of no service ; in their natural habitat, however, originally under- 

 ground shoots or creeping parts, which are covered with earth or leaves, again 

 reach daylight in virtue of the effect of etiolation coupled with geotropism 

 (Lecture XXXIV). Owing to the combined action of etiolation and helio- 

 tropism (Lecture XXXVI), the plant is able to put itself in a position to reach 

 the most suitable light intensity. Etiolation is not induced by absolute dark- 

 ness only ; diminution of light has a similar effect, and generally speaking each 

 varying intensity of light to which a plant is subjected, impresses itself on its 

 structure. As light increases the leaf grows in size up to a certain maximum, 

 while higher illumination causes a diminution in size. The stem behaves con- 



