56 IV. PLASTIDS. 



sap. Let us fix our attention more closely on such a case in the 

 common mullein, Verbascum nigrum. We can examine the 

 petals in water without special preparation, but must remove the 

 adhering air, even if only partially, either by pressure or under 

 the air pump. The epidermal cells of both upper and under side 

 have undulating (sinuous) outlines; the yellow colour of their 

 cell-sap is at once noticeable. The brown spots at the base of 

 the petals are due to a cell-sap coloured from purplish to brown. 

 In the epidermis of the staminal filaments, from which lamellae 

 can be easily cut with the razor, we see a yellow sap ; but 

 besides this there are also in each cell a cinnaber-red irregular 

 lump of colour-material, and a number of colourless leucoplasts 

 filled with starch grains. 



Antirrhinum majus. Similarly it can be at once determined 

 that the yellow-coloured parts of the lower lips of the corolla of 

 Antirrhinum majus (the Snapdragon) contain a sulphur-yellow 

 sap in their cells; the parts coloured red have a rosy cell-sap, and 

 here and there one, seldom more, carmine-red ball of colour- 

 material. 



Coloured Cell-sap. Blue and red petals almost always owe 

 their hue to a coloured cell- sap. A blue cell-sap is found in 

 the epidermis of the corolla of the Periwinkle, Vinca major or F. 

 minor. The epidermis of either side can be readily torn off with 

 the forceps. The epidermal cells, especially of the upper side, 

 are swollen out into papillae. The side walls of the epidermal 

 cells show ridges projecting into the cell cavity (Fig. 19), often 

 swollen at their inner edges, so that they 

 may even spread out into a T-form, and, on 

 account of the stronger refraction of their 

 outer surface and the weaker refraction in 

 the interior, quite give the impression of 

 folds. Upon bent portions of the edge of 

 the petal it can be easily seen that these 

 ridges extend the full height of the cell. 

 FIG. 19. -An epidermal [Si m ii ar r id ges will be found in the meso- 



cell from the under side of L 



the petal of Vinca minor ( x phyll of the leaf of PillUS.] 



Eed cell-sap can be seen in the petal of 



a rose. Here also the epidermis can be readily removed from 

 either side. The upper side has fairly strongly developed papillae, 

 and hence appears so beautifully velvety. The cuticle exhibits 

 strongly marked striation. 



