THTCKENING DEPOSITS '. FIBRE-CELLS. 



397 



Fig. 206. 



Spiral cells of leaf of Oncidium. 



itself in the single layer of the dotted cell — each deposit being 

 deficient at certain points, and these points corresponding with 

 each other in the successive layers — a series of passages is left, by 

 which the cavity of the Cell is extended at some points to its mem- 

 branous wall; and it commonly happens that the points at which 

 the deposit is wanting on the 

 walls of two contiguous Cells, are 

 coincident, so that the membranous 

 partition is the only obstacle to 

 the communication between their 

 cavities (Figs. 203-205). It is of 

 such tissue that the ' Stones ' 

 of stone-fruit, the gritty substance 

 which surrounds the Seeds and 

 forms little hard points in the fleshy 

 substance of the Pear, the Shell of 

 the Cocoa-nut, and the Albumen 

 of the Seed of Phytelephas (known 

 as ' vegetable ivory '), are made 

 up ; and we see the use of this very 

 curious arrangement, in permitting 

 the Cells, even after they have 



attained a considerable degree of consolidation, still to remain 

 permeable to the fluid required for the nutrition of the parts 

 which such tissue encloses and protects. 



291. The deposit some- 

 times assumes, however, 

 the form of definite 

 Fibres, which lie coiled 

 up in the interior of cells, 

 so as to form a single, a 

 double, or even a triple 

 or quadruple spire (Fig. 

 206). Such Spiral Cells 

 are found most abun- 

 dantly in the Leaves of 

 certain Orchid eous plants, 

 immediately beneath the 

 Cuticle, where they are 

 brought into view by ver- 

 tical sections ; and they 

 may be obtained in an 

 isolated state by macerat- 

 ing the leaf and peering 

 off the Cuticle so as to 

 expose the layer beneath, 

 which is then easily sepa- 

 rated into its components. 

 In an Orchideous plant, 



Fig. 207 



fir 



Spiral fibres of Seed-coat of Collomia. 



