182 The Rev. P. Keith on the Internal Structure of Plants. 



of the pistil only excepted. But although it is extended over 

 the whole of the plant's surface, it is not of the same tenuity 

 throughout. In the root and trunk it is in many plants a 

 tough and leathery membrane, or it is a crust of considerable 

 thickness ; while in the leaves, flowers, and tender shoots, it 

 is a fine colourless and transparent film not thicker than a 

 cobweb. It is colourless, however, only when detached ; for 

 when adherent, it assumes the colour of the parts immediately 

 beneath it. Hence the green colour so prevalent in the leaf 

 and tender shoot, and the beautiful variety of hues displayed 

 in flowers and fruits. 



Du Hamel, who seems to have been the first to study its 

 structure minutely, describes it as being formed of a multipli- 

 city of fine and delicate fibres, placed in a parallel direction, 

 but inosculating at regular intervals, so as to constitute a net- 

 work, the meshes of which are filled up with a thin and trans- 

 parent pellicle — single, as in the epidermis of the leaf; or di- 

 visible into several layers, as in the stem of the Paper Birch 

 Betula papyracea, in which you may count six or more*. 



Saussure the elder inspected it as it occurs in the leaves 

 and petals of Jessamine and Foxglove, and describes it as 

 constituting a bark composed of two layers, the interior layer 

 being net-like, and interspersed with a multiplicity of what 

 he calls cortical glands, and the exterior layer being totally 

 destitute of organization f. 



Hedwig describes it as forming a network of fibres that 

 consists of two distinct but adherent laminae ; but he regards 

 the cortical glands of Saussure as being merely pores or 

 apertures perforating the pellicle that fills up the mesh J. 



Comparetti describes it as consisting of a network of fibres 

 ascending in an oblique direction, and forming hexagonal 

 meshes of various sizes and positions, the area of the meshes 

 being occupied by opake or transparent points, of an oval or 

 roundish figure, that seem to be somewhat inflated, as if filled 

 with air or water. He studied it chiefly as it occurs in the 

 leaves of succulent plants §. 



The above descriptions do not, indeed, tally quite so com- 

 pletely as could be wished ; but an exact coincidence was 

 not to be expected in the description of an organ that differs 

 so much in different species of plants, and even in different 

 parts of the same plant. They agree in all that is essential. 

 Whoever will be at the trouble to repeat the observations will 

 find that the foregoing descriptions exhibit a sufficiently cor- 



* Phys. des Arb., liv. i. chap. ii. f Obsei'vations stir P Ecorce de Feuilles. 



X Tracts relative to Botany, 1805. § Senebicr, Phys. Veg. 



