336 Evolution and Adaptation 



Land plants have, in addition to the soft parts, the hard 

 bast and wood which serves the mechanical purpose of sup- 

 porting the soft tissues and protecting them from being 

 injured. The arrangement of the hard parts is such as to 

 suggest that they are the result of the action of pressures 

 and tensions on the plant, for the strongest cells are found 

 where there is most need for them. It is easy to imagine, 

 Nageli adds, that this important arrangement of the tissues 

 is the result of external forces which brought about the result 

 in these parts. 



Nageli accounts for the origin of twining plants as follows. 

 Being overshadowed by other plants, the stem will grow 

 rapidly in the damp air. Coming in contact with the stems 

 of other plants, the delicate stem is stimulated on one side, 

 and grows around the point of contact. This tendency 

 becomes inherited, and the habit to twine is ultimately 

 established. 



The difference in the two sides of leaves is explained by 

 Nageli as the result of the difference in the illumination of the 

 two sides. This influence of light on the leaf has been in- 

 herited. The formation of the tubular corolla that is seen in 

 many plants visited by insects is explained as the result of the 

 stimulus produced by the insects in looking for the pollen.]! 

 The increase in the length of the proboscis of the insect is 

 the result of the animal straining to reach the bottom of the 

 ever elongating tube of the corolla. "The tubular corolla 

 and the proboscis of the insect appear as though made for 

 each other. Both have slowly developed to their present 

 condition, the long tube from a short tube and the long 

 proboscis from a short one." Thus, by purely Lamarckian 

 principles, Nageli attempts to account for many of the adap- 

 tations between the organism and the outer world. But if 

 this takes place, where is there left any room for the action 

 for his so-called perfecting principle ? Nageli proceeds to 

 show how he supposes that the two work together. 



