MEMBRANE. 
9 
myself that they are true perforations by observing 
infusoria pass in and out of the cells in a young plant 
growing in water. 
In process of growth, the thin membranous cell-walls 
become thickened by the deposition of new matter, 
either on their external or internal surface — more 
frequently the latter. The deposit is termed secondary, 
and, according to Schleiden, usually assumes a spiral 
direction. It is generally considered, that if the deposit 
take place before the cell has completed its growth, its 
form will be that of a spiral fibre or band, in conse- 
quence of the cell-wall elongating or growing more 
rapidly than the deposition takes place within the cell ; 
but that, if the deposit take place after the complete 
development of the cell, it may then so happen that 
certain parts being left uncovered by the deposited 
material, pores, or pits having the appearance of pores, 
are produced. We have striking examples of cells 
containing spiral fibres, or bands, in many plants, 
especially the Orchidacece , as in the Pleurothallis , two 
cells from the leaf of which are shown in Fig. 4, e f, 
and in some elongated cells from the stem of a Balsam 
in Fig. 4, b. 
The other form of deposit is more common, and is 
well exhibited in its earliest stage in the two specimens 
of the root of Marchantia polymorpha , in Fig. 3, b, c ; 
or in a, which is a representation of the cells forming 
the pith of the Elder ; the same fact is strikingly 
shown in the plant furnishing the Rice paper, and in the 
