Chap. I.] OF A NERVOUS SYSTEM. 17 



along the line traversed by the stimulus. It has been 

 found that a stimulus radiated from the centre traverses 

 the leaf in a longitudinal more rapidly than it dobS in a 

 transverse direction — a circumstance apparently to be 

 explained by the fact that, in the longitudinal direction, 

 owing to the elongated shape and disposition of the cells, 

 the stimulus has to pass through a smaller number of 

 obstructive cell-walls. 



The irritability and answering movements just described 

 are, however, altogether exceptional events in plant life ; 

 more especially if we refer, as at present, only to cases 

 where there is reason to suppose it possible that the move- 

 ments are in part due to contractility, rather than to mere 

 disturbance of tension in some of the cells — movements 

 of the latter order being not unfrequent in stamens, seed- 

 pods, or other parts of plants. Yet even in these plants, 

 where contractility appears to exist to a more marked 

 extent than in any other known members of the vegetal 

 kingdom, there is no development of a specialized con- 

 tractile tissue, and still less is there an appearance of any 

 nerve fibres along which the molecular disturbance consti- 

 tuting the stimulus may be transmitted. The obstacles 

 opposing the passage of the stimulus, to which reference 

 has been made, would indeed also tend to impede the 

 formation of a special tissue along the line of discharge. 



In Animal Organisms, however, we have a highly im- 

 pressible and very active variety of protoplasm, the units of 

 which, particularly as met with in the lowest forms of 

 animal life, do not go on to the formation of a distinct 

 cell-wall, and are for the most part aggregated into mere 

 Bemi- fluid or gelatinous tissues capable of transmitting 

 vibrations in different directions with the greatest ease. 



This is the case, for instance, in Medusae, which are 

 perhaps the lowest animals in whom a nervous system la 



