STRUCTURE.] APPARENT FIBROUS STRUCTURE PORES. 13 



assistance it would be difficult or impossible to recognise, yet 

 in availing ourselves of such assistance we must keep the 

 anatomical relations constantly in view. The study of these 

 relations leads, I believe, to a result diametrically opposed to 

 that maintained by Mulder and Harting. (See Annals of 

 Natural History, vol. xviii., p. 265, for further details.) 



Elementary membrane generally tears readily, as if its 

 component atoms did 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.) The same 

 circumstance has been remarked by Dr. Willshire in Til- 

 landsia usneoides. (Ann. Nat. Hist, xviii.) Hence it may be 

 conjectured, that what we call primitive membrane is itself 

 the result either of primitive fibres completely consolidated, 

 or of molecules originally disposed in a spiral direction, 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 Dogbanes 

 (ApocynaceaB) and Asclepiads, 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. Mohl, 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 MohPs views, and 

 regards such appearances as caused by the process of ligni- 

 fication. Schleiden goes further, and maintains that all the 

 deposits to which the thickening of membrane is owing have 

 originally a spiral direction. (See p. 9.) 



Membrane is in all cases, when first formed, destitute of 

 visible pores ; although, as it is readily permeable by fluids, 

 it must necessarily be furnished with invisible passages. An 



