IRRITABILITY. 427 



Sachs well says that "Marchantia behaves to intense light 

 like hard steel to a magnet ; the steel, under the influence of 

 the magnet itself becomes a magnet with fixed poles ; Tro- 

 paeolum, on the other hand, behaves to intense light like soft 

 iron to a magnet ; the soft iron assumes a definite but tem- 

 porary polarity which disappears when the influence of the 

 magnet is withdrawn." 



The cases of induced dorsiventrality which have been 

 described all refer to primary shoots. We will now briefly 

 consider the case of lateral branches. We can easily under- 

 stand how dorsiventrality could be induced in them, for they are 

 developed in an oblique position to begin with. It appears 

 that such an induction does, as a matter of fact, take place in 

 the Coniferae which differ in this respect from dicotyledonous 

 forest-trees, the lateral branches of which, as already mentioned, 

 (p. 424), are inherently dorsiventral. Frank found, namely, 

 and his observation has been confirmed by de Vries, that in 

 whatever position the lateral branch of any one of the many 

 Conifers observed was allowed to develope from the bud, the 

 upper surface became the dorsal surface and the lower the 

 ventral. Light seems to take some part in inducing the 

 dorsiventrality, though it is induced in branches grown in the 

 dark, so that probably gravitation is the principal inducing 

 agent in these cases. 



Having now obtained the requisite preliminary inform- 

 ation as to the internal influences which contribute to deter- 

 mine the direction of growth of plant-organs, and as to their 

 properties, we may go on to consider the effect of external 

 stimuli in modifying the direction of growth. 



Radiant Energy. In our consideration of the influence of 

 light upon the rate of growth, it was tacitly assumed that all 

 sides of the growing organ were symmetrically exposed to 

 light. We have now to deal with phenomena, included 

 under the general term Heliotropism, a term which we owe to 

 Frank, which are exhibited when the growing organ is not 

 symmetrically illuminated on all sides. It will be convenient 

 to discuss separately the phenomena exhibited by organs of 

 different physiological properties. 



