IRRITABILITY. 425 



A few instances of this, illustrating the different degrees in 

 which dorsiventrality can be impressed upon the plant, will 

 be of interest. We will begin with an instance of what we 

 may call a slightly dorsiventral organ. Sachs has observed 

 that the young primary shoot of Tropceolum majus is at first 

 orthotropic, and that the exposure of one side of the older 

 internodes to intense light for a time, causes a considerable 

 curvature so that the shoot becomes plagiotropic. It has 

 also become physiologically dorsiventral, the illuminated side 

 being the upper surface, but this is not strongly marked 

 either in its external form or internal structure : it is radially 

 symmetrical and the spiral phyllotaxis remains, but the 

 ventral (inferior) surface has a tendency to form roots which 

 the dorsal (superior) surface has not. Nor has the dorsi- 

 ventrality penetrated deeply into the constitution of the 

 shoot, for the exposure of any side of it, so long as it is still 

 growing, for a time to intense light suffices to cause that side 

 to become the dorsal (superior) surface and the opposite side 

 the ventral (inferior) surface. A similar case, but one in which 

 the dorsiventral nature is more marked, is described by Sachs 

 in the Ivy. The primary shoot of the seedling is, like that 

 of Tropaeolum, at first orthotropic, and radial, but exposure 

 to light on one side causes it to curve so that it is almost 

 horizontal, the illuminated side being, in this position, the 

 uppermost. It is then plagiotropic and exhibits evident 

 dorsiventrality ; the leaves, instead of being spirally arranged 

 as they are in the orthotropic shoot, come to be arranged 

 in two lateral rows, and the inferior (ventral) surface produces 

 rootlets. The dorsiventrality penetrates more deeply into 

 the constitution of the Ivy-shoot than it does in the case of 

 the Tropaeolum-shoot, but it is still capable of alteration : 

 Sachs found, namely, that when a dorsiventral Ivy-shoot 

 was exposed to light on one side for three or four weeks, 

 the illuminated side became the dorsal, and the opposite 

 side the ventral surface, that dorsiventrality was induced 

 in a fresh plane. In the thalloid shoot of Marchantia 

 we have a remarkable case of the induction of dorsiven- 

 trality, and of permanent dorsiventrality. We will trace the 



