INFLUENCE OF EXTERNAL STIMULI. LIGHT 249 



sagittalis. The leaves are greatly reduced, the shoot-axis broadly winged, 

 as appears in the portion of the figure to the right. To the left is 

 shown a lateral shoot which has developed in darkness and in it the 

 wing which starts from the leaf-base appears as only a small rib, and 

 the whole shoot has quite a different look because the increase in surface 

 has not taken place. All plants however, do not exhibit a like reaction. 

 The leaf-like twigs of Ruscus aculeatus, for example, if etiolated through 

 growth in darkness, are smaller than the normal ones but otherwise agree 

 with them in form. 



Amongst Pteridophyta analogous cases are known. In Lycopodium 

 complanatum, which will be referred to again below, the growth in 

 surface extent is dependent upon light. 



With the Cacteae to which we have just referred, we may associate the 

 moss Tetraphis pellucida, which possesses organs of assimilation upon its 

 protonema behaving exactly in the same way. Upon the branched cell- 

 filaments structures arise which have commonly a leaf-like form and are 

 organs of assimilation ; they have a stalk which terminates in a cell-surface. 

 These cell-surfaces are plagiotropous. If the illumination is feeble these 

 cell-surfaces do not develop, but instead there arises an erect richly 

 branched protonema-tuft the filaments of which are divided by longitudinal 

 walls l . Light also may affect the configuration of protonemata in another 

 way. In Diphyscium foliosum the same organs of assimilation are partly 

 flat and leaf-like, partly cylindric, and the difference is probably due to 

 light ; in Sphagnum too the usual flat protonema-forrnation may occasion- 

 ally appear in a form like that of the germ-disk of the Marchantieae 2 . 

 We may here also mention that in etiolated Marchantieae, as well as in 

 Blyttia, and others, the shoots of the thallus when grown in darkness are 

 quite small. 



The large peripheral bladder-like branches which serve the purpose 

 of assimilation in the much-branched and tufted tubes of Codium 3 , one 

 of the Siphonieae, are not developed if the illumination is feeble and 

 thus increase of surface is suppressed. Similarly in Caulerpa 4 the leaf- 

 like organs are only formed in light of a definite intensity, in darkness 

 cylindric organs are produced instead. Sachs has rightly conjectured 

 that the considerable surface development of the vegetative body of the 

 lichens, as compared with that of fungi, is caused by their containing 

 chlorophyll which brings about a different reaction towards the stimulus 

 of light. 



1 Correns, Uber die Brutkorper der Georgia pellucida und der Laubmoose iiberhaupt, in Ber. d. 

 deutsch. bot. Gesellsch. xiii. p. 426. 



2 See Part II of this book. 



3 Berthold, Morphologic und Physiologic der Meeresalgen, in Pringsh. Jahrb. xiii. 

 1 Klemm, Uber Caulerpa prolifera, in Flora, 1893, p. 460. 



