18 



ANxiTOMY AND PHYSIOLOGY OF 



Porous cells of the leaf of Dicra- 



imm glauomn. 



come converted into porous cells. This occurs most remarkably 

 in certain Mosses, especially in the fibrous cells of Sphagnum, 



the leaf-cells of Dioranwu glaucum 

 (%.24), and OdoblepharuTn albidum, 

 &c. (See "Anatomical researches on 

 the Porous Cells of Sphagnum" in 

 •my^VermischteSchriften/' 294; also 

 Schleiden, "Beitrage/'L7l.) This phe- 

 nomenon is very rare in the Phanero- 

 gamia ; I found it decidedly in fibrous- 

 cells, e.g., in the rind of the root of 

 Epidendrum elongatum, in the seed- 

 coat of Martynia, fee, &c. Whether 

 it occurs normally in the wood-cells 

 of Pinus, as linger asserts, is yet a matter of doubt to me. 



In the generality of cases, all the layers deposited on the inside 

 of the primary membrane agree completely in their form, so that 

 there is no reason why we should adopt a further division of the 

 layers than that into primary and secondary membrane. But in 

 particular cases, the secondary membrane consists of two layers of 

 strikingly difierent structure, so that it becomes necessary to dis- 

 tinguish between primary, secondary, and tertiary membranes. 



To what extent such a distinction into secondary and tertiary 

 membrane exists, cannot be stated in the present state of our 

 knowledge. I must, therefore, confine myself to the mention of 

 certain examples in which the existence of the tertiary membrane 

 may be demonstrated with certainty. To these belong the wood 

 cells of Taxus and Torreya, the primary and seconda^ mem- 

 bx-anes of which are formed exactly as in the wood-cells of Pinus, 

 but their cavity is lined with an inner membrane, which is covered 

 with a fibre-like thickening running in regular spiral lines (fig. 25). 



The same structure is repeated in the wood- 

 cells of certain Dicotyledonous trees, e.g., in 

 Viburnum Lantana, 



The contrast between the secondary and ter- 

 tiary membranes is most striking in cells which 

 occur in the coats of the seeds of very various 

 plants, and in wliich one of the inner mem- 

 branes is split into spiral fibres; while the 

 other consists of homogeneous layers, which 

 when wetted with water swell up so strongly 

 that they burst the primary membrane. This 

 property is generally found in the secondary 

 layers, while the tertiary membrane appears 

 as a spiral fibre, e.g., in the outer cells of the seed-coat of Gollomia 

 and other Polemoniacese, of the pericarp of Salvia, in the hairs of 

 the fi:uit of Senecio vulgaris, &c. ; in other cases the secondary 

 membrane is formed of spii^al fibres, and the tertiary layers con- . 



Fig. 25. 



Wood-cells of Taixfus 

 hacoata. 



