Royal Society. 455 



ones, which freely anastomose with one another, and then either are 

 continued into the enamel, or terminate at the boundary between 

 these two substances. Various modifications of this structure, ex- 

 hibited in the teeth of different animals, in the class Mammalia and 

 Fishes more particularly, are minutely described. The granular sub- 

 stance appears to be composed of irregularly shaped osseous gra- 

 nules, imbedded in the same kind of transparent medium which ce- 

 ments the tubuli together. External to the granular portion, the 

 author finds another substance entering into the formation of the 

 simple tooth, and commencing where the enamel terminates ; and 

 which he describes as beginning by a thin and transparent layer con- 

 taining only a few dark fibres, which pass directly outwards ; but 

 assuming, as it proceeds towards the apex of the fang, greater 

 thickness and opacity, and being traversed by vessels. 



External to the enamel, and in close connexion with it, in com- 

 pound teeth, is situated the crusta petrosa, a substance very similar 

 to the bony layer of the simple tooth. It contains numerous cor- 

 puscles, and is traversed by numerous vessels entering it from with- 

 out, and anastomosing freely with one another, but terminating in 

 its substance. These investigations of the structure of the different 

 component parts of teeth, furnish abundant evidence of their vascu- 

 larity and consequent vitality. 



" On the evolution of Nitrogen during the growth of plants, and 

 the sources from whence they derive that element." By Robert Rigg, 

 Esq. Communicated by the Rev. J. B. Reade, M.A., F.R.S., &c. 



In this communication the author follows up his inquiry into the 

 influence and importance of nitrogen in vegetable physiology, by 

 noticing, in the first place, the experiments of Dr. Daubeny, M, De 

 Saussure, Sir Humphry Davy, and those which he himself has made ; 

 all of which tend to prove that nitrogen is evolved during the healthy 

 performance of the functions of plants ; that the proportion which 

 it bears to the oxygen given off is influenced by the sun's rays ; but 

 that owing to the necessary exclusion of the external atmosphere 

 during the progress of the experiments, it is impossible, with any 

 degree of accuracy, to calculate the volume of these evolved gases 

 during any period of the growth of plants in their natural state. 



If to this indefinite quantity of nitrogen given off by plants there 

 be added that definite volume incorporated into their substance and 

 shown in the author's former tables*, the question arises, whence do 

 plants derive their nitrogen, and does any part of it proceed from 

 the atmosphere ? A problem which the author proposes to solve by 

 a series of tabulated experiments upon seeds, and seedling plants, 

 indicating a large excess of nitrogen in the latter, and under such 

 circumstances of growth that he is compelled to fix upon the at- 

 mosphere as its source. 



By the same mode of experimenting, the author attempts to show 

 that the differences which we find in the germination of seeds and 

 the growth of plants in the shade and sunshine, are apparently due 

 in a great measure to the influence of nitrogen. And he concludes 



• See the present volume, p. 223. 



