DORSI- VENTRALITY. 



491 



The flat shoots of the Liverworts exhibit a dorsi-ventrality like that of the 

 foliage-leaves, which they resemble in other respects also ; and the same is the case 

 with many Algae and the prothallus of the Fern. In all these cases the contrast 

 between the upper and lower (or dorsal and ventral) side is still more peculiarly 

 characterised by the nature of their off-shoots: the roots always arise from the 



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lower or ventral side, which is turned away from the light, while the sexual organs of 

 the dorsi-ventral Liverworts arise from the upper side; those of the prothallus 

 of the Fern from the lower side, which is turned from the light. 



The relations are more complicated when a shoot possesses dorsi-ventral 

 organisation, and at the same time a sharp segmentation into axis and leaves ; 



