414 VEGETABLE PHYSIOLOGY 



The co-ordination of these factors we have seen is one 

 of the most marked features of a highly differentiated 

 nervous system. In this respect we cannot note anything 

 in the plant which in its elaboration or in its peculiar 

 efficiency can be compared with the co-ordinating mechanism 

 of animals. Certain responses to stimulation can be effected, 

 but no definite regulation of any function shows any great 

 completeness. We have seen this particularly in the case 

 of the influence of temperature. Though a certain range 

 of temperature is imperative for the plant's well-being, it 

 has no power, or but little, to co-ordinate its own produc- 

 tion or expenditure of heat with the variations of tempera- 

 ture to which it is exposed. 



Nor can we observe any structural differentiation in the 

 direction of such co-ordination, though very complex move- 

 ments in some cases point to its existence. We may 

 instance the assumption of the nyctitropic position by the 

 leaves of Nicotiana and of certain Leguminosce. The 

 plant shows an almost complete absence of the differentiation 

 which reaches its highest point in the nerve-cell. There 

 is apparently no co-ordinating mechanism which receives 

 the impulses from the sense-organs, and initiates in conse- 

 quence the resulting movement. One case only has so far 

 been put on record which even suggests a complexity of 

 this kind. Attention has been called by Darwin to a 

 peculiarity in the behaviour of the tentacles of Drosera, 

 in which something of this nature is seen. When one of 

 these organs is stimulated, its actual bending is preceded 

 by a curious motility of the protoplasm of the cells of its 

 stalk which has been called aggregation. If a tentacle on 

 the surface of the leaf is excited, the tentacles of the margin 

 are gradually inflected towards the excited spot. If the 

 cells of one of these marginal tentacles are watched during 

 the experiment, their contents are found to undergo this 

 aggregation, but those nearest its apex manifest it first. 

 If the aggregation were the direct effect of the stimulus, 

 those which it reached first, i.e. those nearest the base of 



