166 Dr. H. Johnson on a Property iri Plants 



their own weight. Fig. 2. a, represents the stem in its pro- 

 per, b in an inverted position. 



a Fig. 2. b 



By experiments similar to the preceding, I have detected 

 this property in above seventy different genera of plants, a 

 table of which is now beside me, but which would occupy 

 too much room to be here inserted. 



Divergence being thus proved to exist very generally in 

 plants, let us, in the next place, endeavour to learn something 

 of its nature and cause. 



After carefully considering all the observed facts, I am led 

 to conclude, that they must depend, either on physical elasti- 

 city, or on that vital contractile power which is called irrita- 

 bility. No other known principle suggests itself to which 

 I can reasonably ascribe them. The following facts prove, 

 I think, very clearly that the phaenomena of divergence are 

 not due to elasticity. 



1. The woody parts of trees, and even the rattan cane, 

 which are certainly some of the most elastic vegetable sub- 

 stances, never exhibit divergence on division*. 



2. The stems of many plants which in their recent and 

 growing state are divergent on division, lose this property 

 when they become dead and dried, although they are in the 

 latter case much more elastic than before. For example, the 

 stem of the Common Teasel {Dipsacus fullonum\ which is 

 strongly divergent in its recent green state, loses this property 



[• A botanical friend suggests the inquiry whether Dr. Johnson has ever 

 tried the effect of division on Dirca palustris, or any plant of the natural 

 order ThymeUece ? — Edit.] 



