1C8 PHYSIOLOGY OF NUTRITION. 



as much Potassium iodide as can be absorbed, and finally saturate 

 the solution with Iodine. 



We can easily convince ourselves of the existence of the cuticle 

 by study of a transverse section of a one-year-old stem of Viscum 

 album, or of a leaf of an Aloe. The cuticle is also very strongly 

 developed on the surface of the leaf of Ilex aquifolium. A very 

 delicate transverse section through the midrib of an Ilex leaf 

 teaches us that the epidermal cells of the under side of the leaf 

 have a half-moon shaped lumen. The cuticularised layers of the 

 cell membranes extend into the side walls of the cells, and are 

 covered on the outside by the cuticle proper. 1 The cuticle of most 

 leaves and of other parts of plants is thin and very delicate in 

 constitution. 



To familiarise ourselves with cork tissue, we may examine 

 delicate transverse sections of the skin of a potato, of an ordinary 

 bottle cork, or of one of the older stems (say, 1 cm. thick) of a 

 plant of Aristolochia Sipho. The cork cells, arranged in radial 

 rows, are more or less tabular in form. On studying the periderm 

 of Aristolochia, it is seen that broader zones of wide cork cells, and 

 thinner zones of narrow cork cells, alternate with one another. 

 Concentrated potash solution stains both cuticularised and corky 

 membranes yellowish. This coloration is intensified by warming 

 the preparations. 2 



We prepare a transverse section of a twig of Tilia a few mm. 

 in thickness. The microscopical structure is easy to understand. 

 We are here chiefly concerned with the wood and the bast of 

 the vascular bundles, the former being composed principally of 

 vessels of varying diameter and wood fibres. The bast masses 

 taper in a wedge-like manner, the apices of the wedges being 

 directed towards the cortex, while the wedge-shaped ends of the 

 primary medullary rays, which alternate with the masses of bast, 

 turn their apices towards the wood. In the bast masses bright 

 strips, composed of very much thickened bast fibres, alternate with 

 dark strips of soft bast. We now lay our section on the slide, in 

 a drop of alcoholic phloroglucin solution. After a little time, when 

 the alcohol has evaporated, we moisten the section with concen- 

 trated Hydrochloric acid, and examine it under the microscope. 

 All the ligriified elements have stained red, while the unlignified 

 elements remain unstained, so that phloroglucin forms an ex- 

 cellent reagent for woody material. 3 It is particularly to be 

 observed that not only the elements of the wood proper of the 



