114 VIII. THE MONOCOTYLEDONOUS STEM. 



to have approximately the same structure. Either of these 

 plants can, of course, be readily grown for use. 



Vascular Bundle in Iris. Now take cross and longitudinal 

 sections of a fully-developed leaf of Iris florentina, or other Flag 

 Iris, preference rjeing again given to alcohol material, because it 

 is more easy to obtain good sections, it contains no air, and 

 besides this, the cell-contents are fixed, so that we can more 

 readily study them. Section-cutting will be facilitated if the 

 material is previously laid in a mixture of alcohol and glycerine. 

 Lay the sections for a few hours in borax-carmine ; then treat 

 them for a short time with methyl-green. The cell-contents 

 have taken up carmine, which in the form of borax-carmine does 

 not stain the cell- walls, while the lignified walls are stained green 

 with methyl-green. The vessels appear stained green, and usually 

 also the outer elements of the sheath, i.e., those impinging on 

 the bast of the bundle. Besides this, a group of elements with 

 swollen walls, the protophloem, are noticeable in the outer region 

 of the bast through their blue colour. We will therefore com- 

 mence with the study of such a preparation, from which Fig. 42 

 is drawn. In this figure all the cells which are especially rich 

 in protein contents, and therefore are conspicuous from their 

 carmine colour, are shaded ; the green stained walls of the vessels 

 are, on the other hand, represented darker in the figure ; while 

 the protophloem elements coloured blue are left clear. The 

 thickened elements of the ground-tissue bounding the bast, when 

 the section is taken through the base of the leaf, are still unligni- 

 fied, and therefore remain unstained. To rapidly stain a section, 

 it can be treated with methyl-green alone ; the colour of the cell- 

 contents as here described is then absent. If the methyl-green 

 is to stain only the lignified cell-walls, the exact time for staining 

 must be carefully watched, or, if over-stained, the cellulose walls of 

 the ground-tissue can be decolorised by rapidly washing in alcohol, 

 best under the low power of the microscope, stopping the process 

 at the right moment by transferring the section to water. We 

 proceed to examine the bundle from the wood towards the bast, 

 and therefore from the upper surface of the equitant leaf, turned 

 inwards, towards the lower surface, turned outwards. We first 

 note that the number of vessels, or vascular tracheides, in the 

 wood is pretty considerable, and that their width diminishes 

 towards the bast. The vessels are in direct contact with one 

 another, or else are separated by the slightly thickened compara- 



