88 



VI. THE EPIDERMIS STOMATA. 



walls of the epidermal cells ; it is caused by the razor in cutting, 

 and not infrequently recurs with hard elastic objects. 



A section treated with chlorzinc iodine, shows the highly re- 

 fractive thickening layer coloured yellow-brown ; it is, therefore, 

 cuticularised. The inner covering to this layer (i) is, on the other 

 hand, coloured violet, as likewise are the other cell- walls of the leaf. 

 The yellow-brown coloration passes over the " hinge " on to the 

 outer and inner ridges which are on the guard-cells. Elsewhere the 

 guard-cells are coloured violet. On treatment with concentrated 

 sulphuric acid, the whole of the part which colours yellow-brown 

 with chlorzinc iodine remains undissolved for a time, but after 

 some hours' action this also is dissolved, and then the delicate 

 cuticle, and the fine middle lamellae found between the epidermal 

 cells, alone still persist. The cuticle is continued over the guard- 

 cells to the junction with the chlorophyll-containing inner cells. 

 The cuticular layers and the cuticle take a brown colour in the 

 sulphuric acid. The oil present in the guard-cells "balls" 

 together, immediately on the entrance of the acid, into a highly 

 refractive spherule, which disappears after some time. In sec- 

 tions laid for a few hours in a solution of alkanet in 50 per cent, 

 alcohol the cuticle becomes red ; any fats present take a deeper 

 red colour, so that this coloration of the cuticle is taken to show 

 the presence of fatty substances in it. 



Water-Pores of Tropceolum. We will now turn our attention 



to Water- Pores orWater- 

 Stomata, These show 

 the same structure as the 

 air-stomata, but are larger, 

 and the cleft, as well as 

 the underlying intercellu- 

 lar space, is, at least in 

 part, filled with water. 

 The guard-cells of these 

 stomata as a rule quickly 

 die, and then lose their 

 movability, so that the 

 cleft between stands wide 



open. The most favour-- 

 FIG. 32. Water- stoma at the edge of the leaf f 

 of Tropceolum majus, together with the surround- able object lor the study 



of these water-pores is 

 Tropceolum majus (the Indian cress or so-called " Nasturtium "). 



