DORSI-VENTRAL STRUCTURE DUE TO LIGHT. 535 



on the shaded side. Leitgeb demonstrated this highly important fact very elegantly 

 by sowing' the spores of a Fern, Ceratopteris thalictroides (which, moreover, grows in 

 very wet places) on the surface of a clear nutritive solution, where they produce 

 floating prothallia. If the sowing is illuminated from above, the archegonia and 

 root-hairs arise on the under side — the latter also from the margins — and both grow 

 down into the liquid ; if the illumination takes place from below, however, the 

 prothallia grow downward into the liquid, but become curved, as soon as they begin 

 to develope the flattened surface, so that the latter is directed at right angles to the 

 incident light. Here both the surfaces of the plate of tissue ^re in the water, and 

 nevertheless the archegonia and roots are developed only on the side turned away 

 from the light. Prantl has extended this experiment with the prothallia of other 

 species, and varied it in many ways, with reference to which, however, I cannot 

 here go more into details. I will only lay stress on one point, that not only the 

 dorsi-ventrality but also the extension into a bilateral thin plate of tissue is deter- 

 mined by the light, and that such a plant rendered bilateral and dorsi-ventral by the 

 light also places itself with its surfaces at right angles to the incident light. 



Those Liverworts which consist of flat shoots behave also exactly like the 

 prothallia ; in them also, as Mirbel partly demonstrated with Marchantia about fifty 

 years ago^, the dorsi-ventral organisation and bilateral extension are induced by 

 the light. The germination of the spores of Marchantia runs its course very 

 much as in the case of the Ferns described above, and if they are sown on the 

 surface of soil or damp turf, and care is taken that they are lighted from one 

 side only, it is found that the young, approximately heart-shaped plantlets extend 

 their surfaces all at right angles to the direction of the incident light, although 

 their axes of growth may assume the most various positions. The root-hairs 

 appear exclusively on the shaded side, the stomata only on the upper side. It 

 is still easier to make use of the gemmae of Marchantia for experiments of this 

 kind, for example by letting them float on an aqueous solution of nutritive 

 substances in a transparent glass vessel; if the water is illuminated from below 

 by means of a large mirror (for which however very intense light must be 

 employed) while the upper side of the vessel is covered with an opaque lid, the 

 gemmae grow in the usual manner, and produce broad ribbon-like shoots floating 

 on the surface of the water, which develope their root-hairs chiefly upwards and 

 away from the light, and their stomata on the under side which is turned towards 

 the light. The whole significance of this action of light is only apprehended 

 however on noticing (see Fig. 341, which represents apiece of a transverse section 

 at the margin of a shoot) that the structure here concerned consists of several 

 layers of cells, the relations of growth of which are very complicated, and 

 that the whole of the organisation, represented in the figure in outline only, can 

 be brought by the influence of the light into a completely inverted position. If 

 the Fig. 341 for example is supposed to be illuminated from below, the picture 



' Mirbel's statements occur in his celebrated work, ' Recherches anatomiques et physiologiqties 

 sur k Marchantia folymorpha'' (Mem. de I'acad. d. scienc. de I'institut de France, 1835). These 

 important statements of Mirbel's remained unnoticed till I induced Pfeffer in 1870 to test them, and 

 where possible to study the phenomena more closely. The latter published his results in the 'Arb. 

 des bot. Inst, in IVzbg.' B. I, p. 77. 



