TISSUES OF PHANEROGAMIA 693 



that the original cell- wall there remains imthickened. A more 

 complete consolidation of cellular tissue is effected by deposits 

 of sclerogen (a substance which, when separated from the resinous 

 and other matters that are commonly associated with it, is found 

 to be allied in chemical composition to cellulose) in successive 

 layers, one within another (fig. 529, A), w r hich present them- 

 selves MS concentric rings when the cells containing them are cut 

 throng] i ; and these layers are sometimes so thick and numerous 

 us almost to obliterate the original cavity of the cell. Such a tissue 

 is known as sclerenchyme or sclerenchymatous tissue. By a con- 

 tinuance of the same arrangement as that which shows 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 succes- 

 sive layers a series of passages is left, by which the cavity of the 

 cell is extended at some points to its membranous wall ; and it 

 commonly happens that the points at which the deposit is wanting 

 011 the walls of the contiguous cells are coincident, so that the 

 membranous partition is the only obstacle to the communication 

 between their cavities (figs. 529-531). 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 endosperm of the seed of Phyt- 

 elephas (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. 



The deposit sometimes assumes, however, the form of definite 

 fibres, which lie coiled up on the inner 

 surface of the cells, so as to form a single, 

 a double, or even a triple or quadruple 

 spire (fig. 532). Such spiral cells are found 

 abundantly in the leaves of certain orchi- 

 daceous plants, immediately beneath the 

 epiderm, where they are brought into 

 view by vertical sections; and they may 

 be obtained in an isolated state by mace- 

 rating the leaf and peeling off the epiderm 

 so as to expose the layer beneath, which is 

 1 1 ieii easily separated into its components. FIG. 532. Spiral cells of leaf 

 In an orchidaceous plant named Saccola- of Onddium. 



biutn guttatutn the spiral cells are unusu 



ally long, and have spires winding in opposite directions, so that by 

 their mutual intersection a series of diamond-shaped markings is pro- 

 diu-cd. Spiral cells are often found upon the surface of the testa or 

 outer coat of seeds ; arid in Collomia grandiflora, Salvia verbenaca 

 (wild clary), and some other plants, the membrane of these 

 cells is so weak, and the elasticity of their fibres so great, that 

 when the membrane is softened by the action of water the fibres 

 suddenly uncoil and elongate themselves (fig. 533), springing out, 

 as it were, from the surface of the seed, to which they give a 



