4^4 LECTURE XVIt. 



downwards and most primary shoots vertically upwards, 

 whilst lateral branches, both of roots and shoots, are usually 

 inclined to the vertical. We see at once that the response of 

 different organs to the action of external influences is by no 

 means the same, but that each responds in a manner peculiar 

 to itself. The organs, to use Sachs' apt expression, are 

 anisotropic, that is to say, they are endowed with different 

 kinds of irritability and in different degrees. On this basis, 

 we can at once classify all organs into two great groups ; 

 those, namely, which grow more or less nearly vertically, 

 either upwards or downwards, and those which grow inclined 

 at a greater or a smaller angle to the vertical. The organs of 

 the former group are termed, in accordance with Sachs' pro- 

 posed nomenclature, ortho tropic, those of the latter plagio- 

 tropic. To the former group belong the great majority of 

 organs which are physiologically radial, as well as isobilateral 

 organs such as the Iris-leaves ; to the latter belong all organs 

 which are dorsiventrally organised. If we now enquire into 

 the causes which determine the position of a plant-organ, 

 we find that it must depend upon its peculiar irritability, and 

 upon a certain balance between the responses given by the 

 organ to the various directive influences which act upon it. 

 The orthotropism of radial and of isobilateral organs, and 

 the plagiotropism of dorsiventral organs depends in each 

 case upon the peculiar irritability associated with their nature, 

 and upon a certain relation between the responses which they 

 give to the action of light, of gravity, etc. 



We may, before leaving the subject, briefly enquire into 

 the causation of these different kinds of physiological nature. 

 In some cases they can only be accounted for by regarding 

 them as inherent in the constitution of the organ. This 

 appears to be the case, for example, with regard to the radial 

 and isobilateral organs, and also with regard to certain dorsi- 

 ventral organs, such as most leaves, and lateral branches, 

 particularly those of forest-trees in which, according to Sachs 

 and to Frank, the dorsiventrality is entirely due to internal 

 causes. In other cases dorsiventrality is induced under the 

 influence of external conditions, more particularly of light. 



