426 LECTURE XVII. 



development of the thalloid shoot which, for the sake of 

 brevity we will term the thallus from one of the gemmae of 

 the plant. The gemma is a small discoid lenticular mass of 

 parenchymatous cells, with two diametrically opposite depres- 

 sions in its lateral margins. It is bilaterally symmetrical, and is 

 not dorsiventral ; but whichever of the two surfaces happens to 

 come into contact with the soil when the gemma is sown 

 becomes and remains the ventral surface, whereas the upper 

 surface becomes and remains the dorsal. The internal struc- 

 ture, at first homogeneous throughout (see Fig. 49) undergoes 



FIG. 49 (after Pfeffer). Gemma of Marchantia as seen in transverse section, the 

 plane of section being across from one lateral depression (growing-point) to 

 the other, a, cells capable of developing ropt-hairs ; e, growing-point ; d, 

 the margin of the gemma projecting on the further side of the depression. 



considerable modification as the gemma developes into the 

 thallus, presenting, towards the upper surface the characteristic 

 air-chambers ; the lower surface bears a large number of root- 

 hairs. The thallus is then distinctly dorsiventral. The 

 induction of dorsiventrality may probably be partly due, as 

 Pfeffer suggested, to the action of gravity and to the effect of 

 contact with the substratum, but is chiefly due, as Zimmer- 

 mann has clearly shewn, to the influence of light. It is, under 

 all circumstances, the shaded side of the gemma which 

 becomes the ventral surface of the thallus, and conversely it is, 

 under all circumstances, the illuminated side of the gemma 

 which becomes the dorsal surface of the thallus. When once 

 induced, the dorsiventrality of the Marchantia-thallus appears 

 to be irreversible. In comparing Marchantia with Tropaeolum 



