4 
ORGANOGRAPHY. 
BOOK 1 
cells below the stomates of Pinus sylvestris l*^)? and 
there are sufficient traces of it to be found elsewhere to justify 
the opinion that it is a common mode of increment in thickness. 
Turpin has remarked that this thickening of the membranous 
sides of cells by means of a hard sedimentary matter, called by 
him Sclerogen^ is what causes the grittiness of the pear, and the 
boniness of the stone of the peach and plum, in all which the 
osseous parts were originally membranous. It is, however, 
by no means in old or woody parts that a thickening of the 
membrane takes place : it may be observed distinctly in the 
cells of the corolla of Convolvulus tricolor, and in all proba- 
bility occurs in any part containing fluid matter exposed to 
decomposition. 
Elementary membrane generally tears readily, as if its 
component atoms do not cohere with greater force in one 
direction than another; but I have met with a remarkable 
instance to the contrary of this in Bromelia nudicaulis, in 
which the membrane of the cuticle breaks into little teeth of 
nearly equal width when torn. (Plate I. fig. 6.) Hence it 
may be conjectured, that what we call primitive membrane is 
itself the result either of primitive fibres completely conso- 
lidated, or of molecules originally disposed in a spiral direc- 
tion, as Raspail supposes. [Chim. Org. p. 85.) 
In the membrane of certain plants, as in the liber of the 
Oleander, in Vinca minor, and others belonging to the 
families of Apocynaceas and Asclepiadaceae, an appearance is 
discoverable of spiral steep ascending lines, some of which 
turn to the right, others to the left, thus dividing the surface 
into a number of minute rhomboidal spaces. Mold, however, 
who has made this observation, does not therefore consider 
with Grew that the membrane is woven together of fibres, 
but that their appearance is owing to a small difference in 
the thickness of the cellular membrane : “ Perhaps a different 
arrangement of the molecules at various points, perhaps a 
small difference in the thickness of the membrane, causes a 
different refraction of light, precisely in the same way as 
fibres are visible in badly melted glass.” Valentin confirms 
Mohl’s views, and regards all such appearances as caused by 
the process of lignification. 
It is in all cases destitute of visible pores; although, as it is 
