LIGHT AND GRAVITATION, AND DORSI-VENTRAI. STRUCTURE. 527 



Moreover, a branch - system consisting of simple cell-filaments can also 

 behave towards the light like the flat shoot of Marchantia, which consists of 

 continuous tissue. If the spores of one of our commonest Mosses, Fiinaria 

 hygro77ieirica, are sown on a brick-shaped piece of damp turf which is artificially 

 saturated with nutritive substances, and care is taken that the light for weeks and 

 months always falls only on the same side, there is developed from the spores 

 the protonema, Fig. 343, already described above (p. 30) ; the filaments of the 

 primary shoot of this creep on the turf like rhizomes, putting forth roots below and 

 producing on their upper side erect branched filaments containing chlorophyll, from 

 which again secondary and tertiary lateral shoots arise. It is shown now that 

 these upright branch-systems become extended in a plane at right angles to the 

 incident light, and I have carefully convinced myself of the fact that this is due 

 not simply to heliotropic curvatures and torsions, but that the bi-serially arranged 

 offshoots come forth from their parent axes strictly right and left. In other 



Fig. 343.— Protonema of Funm-ia hygrometrica. li a creeping- main shoot, from which arise the lateral shoots, 

 containing chlorophyll and branched in two series. K a bud which will develope into a Moss plant ; tu its first root. 



words, the light in this case causes the growing points of the lateral shoots to 

 arise only on the flanks of the mother-shoots, if these are always illuminated from 

 one side^ 



Supposing now that the lateral shoots which arise from the filament b in 

 the figure, stand so close together' as to be everywhere in contact, we should 

 then obtain a leaf-like surface of tissue developed at right angles to the incident 

 light. So it is however as a matter of fact with the flat shoots of Marchantia, 

 and with those of the Opuntia ; they become narrow and filamentous in the dark, 

 but in intense light falling from one side develope their normal flat form, in such 

 a manner that the surfaces stand at right angles to the incident rays. 



So far it is gravitation and light which have produced the described efi'ecis 



' The relation here described between the protonema of Fiiuaria hygroDutrica and light was 

 first cited in my treatise 'Über orthotropc und plagioi rope Pßanzcutlicilc' (Arb. des bot. Inst, in 

 Wzbg. B. II, p. 256). I have completely set aside the doubt I there expressed, by means of later 

 investigations, and thus established the result expressed in the text. 



