346 On Dendritic Figures. 



bodies of certain corallines assume an aborescent character, as 

 in the Corallina muscosa of Ellis. (See Plate 2 of his work.) 



The next examples of arborescent evolutions occur in the so- 

 lid woody frame-work of the leaves of trees, as displayed after 

 the membranous or parenchymatous substance has been re- 

 moved by maceration ; and a remarkable example of an accom- 

 modated structure of leaves happens in the Ranunculus aqua- 

 tilts, in which the floating leaves possess an entire covering of 

 skin, while the submersed leaves are subdivided like those of 

 fennel, as if the water had stopped the evolution of the skin 

 rendering the organ more like the gills of fishes. 



For the better understanding of physiological, and conse- 

 quently of pathological, phenomena, it is very important to dis- 

 tinguish between physical causes of general influence, and the 

 especial or peculiar causes termed vital, which belong conjointly 

 to organized living bodies ; and the facts now submitted mvist, 

 I believe, lead to more exact and practical discriminations as to 

 the causes of embryotic evolution, the growth of organized 

 parts, the reparation of laesions, and morbid deviations from 

 natural structure. 



If it be granted that arborescing vessels are only gross ac- 

 commodations or appliances of convenience in animal function, 

 and that they always originate under physical direction, and 

 not from a vital or mysterious necessity, we may assume to 

 have made one step further in natural knowledge. 



These assumptions may, however, be justly supported by the 

 unquestionable existence of entire living distinct animals and 

 vegetables, devoid of arborescing vessels or ramifying fibres. 

 The former occur in dropsical fluids and in uncysted tumours, 

 whicli are termed globular hydatids ; the latter, in the Tremella 

 nostoc. These hydatids are so far parasitical that they exist 

 only in the natural fluids of living animals. The Tremella 

 nostoc has probably a parasitical origin, since it always ap- 

 pears upon moist and decayed wood, or on dead leaves in the 

 spring season. 



London, Langham Place., 

 October 1. 1838. 



