46 PERMEABILITY PERFORATION. [BOOK i. 



impermeable, as if they were made of glass. Probably the 

 suggestion of Mr. Thwaites (page 43), that their sides should 

 be regarded as being spongy, is a near approximation to the 

 truth. An opinion different from this has been entertained 

 by some observers, who have described and figured perfora- 

 tions of the membrane in various plants. Mirbel formerly 

 stated that te the sides of the bladders are sometimes riddled 

 full of holes (fig. 4), the aperture of which does not exceed 

 fiff- 4 - the -g-i-o- of a millimetre (or of half a line) ; or 



are less frequently pierced with transverse slits, 

 which are occasionally so numerous as to trans- 

 form the bladders into a real articu- fig. 5. 

 lated tissue, as in the pith of the 

 Nelumbium (fig. 5)." This state- 

 ment is now well known to have 

 been founded upon inaccurate ob- 

 servation ; what the supposed pores really are has 

 already been explained. (See page 18). 



The only case of undoubtedly perforated parenchym with 

 which I am acquainted is in Sphagnum, where it was first 

 noticed by Mr. Valentine (Muscologia Nottinghamiensis, No. 1. 

 1833). He correctly describes this genus as having the exterior 

 cells of its branches furnished with an aperture communicating 

 with the external air. f( The aperture is tolerably distinct in 

 S. acutifolium ; it is situated at the upper end of the cell, and 

 stands off obliquely, appearing like a minute truncated cone. 

 An easy way to observe it is, to press out the air contained in 

 the cells, which escapes from the aperture in a minute bubble/' 

 This curious contrivance might have been supposed analogous 

 to the air passages into the trunk, below the insertion of the 

 leaves of Tree Ferns, if it did not equally exist in all the 

 parenchym of the leaves themselves. Mr. Valentine does not 

 notice the latter fact, and I believe he considers the circles in 

 the leaves of Sphagnum not to be apertures : but I had ascer- 

 tained, by Mr. Readers ingenious charring process, that they 

 undoubtedly are openings, before I saw John Roper's paper 

 upon the subject in the Annales des Sciences (n. s. x. 314). 

 This writer proved that the circular spaces in Sphagnum 

 leaves are openings, by observing the exit and entrance 



