310 METAMORPHOSIS 



light intensity, the origin or development of an organ may be stopped by 

 insufficient illumination, or may be guided in other directions, so as to present 

 in the long run a totally different appearance. The Fungi form good examples of 

 this. The formation of sporangia does not take place in the dark in Pilobolus 

 microsporus, while the sterile sporangiophore grows far beyond the normal 

 length, and an analogous phenomenon inCoprinuswas referred to above (GRANTZ, 

 1898). Let us glance next at the behaviour of the roots in the higher plants. 

 In many cases these develop only in darkness, and hence aerial roots may appear 

 in etiolated plants, roots which are entirely wanting in plants when exposed 

 to light, even though they be cultivated in a damp atmosphere. On the other 

 hand, light stimulates the formation of buds and thus (WIESNER, 1895, 685) buds 

 are produced on the upper side of the downwardly-bent branches of the willow, 

 which are mostly illuminated from above, and on the underside of the upright 

 branches of the poplar, which are more illuminated from below. Very remark- 

 able effects of light have been observed in the case of subterranean shoots. The 

 offsets of Circaea form scale-leaves in the dark but foliage-leaves in the light 

 (GoEBEL, 1880) ; the stola of Adoxa may be made to grow greatly in length if 

 exposed to light, while darkening them at once induces a stoppage of longitudinal 

 growth and a formation of tubers (SxAHL, 1884). Further, the development of 

 potato tubers is associated with darkness, and it is possible by appropriate 

 means to induce the formation of tubers even on the top of the leafy shoot 

 (V6CHTING, 1887). Perhaps the most interesting case is that described by 

 BERTHOLD (1882) ; he was able by weak illumination to induce the formation 

 of roothairs from the apex of the shoots of many Algae (Callithamnion, Bryopsis). 

 In Bryopsis the pinnae also were transformed into roots in the dark (NoLL, 

 1888 and 1900 ; WINKLER, 1900 a). 



We shall have another opportunity of studying the effect of the intensity 

 of light on flower formation, at present we have still to glance at the influence of 

 the direction of light on the structure of organs. Unilaterally incident light, 

 i. e. illumination of different parts of the plant with light of unequal intensity, 

 often produces formative results well worthy of notice. Very frequently in 

 plants which have developed polarity, light-direction determines which end 

 is to be base and which apex, which root and which shoot. In Equisetum, 

 according to STAHL (1885), the first plane of division in the germinating spore 

 is formed at right angles to the path of the incident ray, thus separating a shaded 

 root-cell from the illuminated prothallus-cell. WINKLER (1900 b) has observed 

 a similar phenomenon to occur in the egg -cells of Cystoseira barbata. He was 

 also able to determine the time required to develop this result ; thus unilateral 

 illumination for three hours had no effect, but four hours' exposure was sufficient 

 to produce a result after the plant had been many hours in darkness. Germination 

 and a differentiation of root and shoot also occurred, though delayed, without 

 any such unilateral illumination. Many lower plants behave in the same way, 

 while in others, and especially in the higher plants, polarity is undoubtedly 

 independent of external conditions. 



The symmetry of the plant body is also dependent on the direction of light, 

 being radial when the light falls equally on all sides and dorsiventral when the 

 light is unilateral. Thus in Antithamnion cruciatum the branches are approxi- 

 mately decussate in diffuse light, but if illuminated on one side only they all 

 arrange themselves in one plane perpendicular to the path of the incident ray. 

 Other structures are always dorsiventral and the light determines only which 

 side shall be the upper, which the lower. Examples of this are seen in the 

 shoots of Lepismium radicans and Hedera helix, where roots are formed on the 

 shaded side only ; the rhizome of Caulerpa forms roots on the shaded and shoots 

 on the illuminated side, in fern prothalli roothairs and sexual organs are formed 

 on the shaded side, and in the thallus of Marchantia rhizoids are formed under- 

 neath and assimilatory parenchyma above. The direction of the light determines 



