54 IV. PLASTIDS. 



moved to the side walls of the cells, leaving the upper and under 

 walls free (position of Apostrophe). The former is the normal posi- 

 tion in daylight, the latter at night. A short time, often less than 

 an hour, in diffused light, suffices to change the position in the 

 previously darkened plants from that of Apostrophe to that of 

 Epistrophe. In the latter position the chlorophyll bodies, which, 

 as already seen, are lenticular, present their broad sides to the 

 ligfyt, and therefore assume the best position for effective assimi- 

 lation. If a strong tuft of the moss is placed in bright sunshine, 

 and protected from too high temperature by a water screen, the 

 chlorophyll bodies again pass to the side walls. 



Orange Chromatophores of Tropceolum. In order to become 

 acquainted with chromatophores of other colours, let us turn 

 next to Trop&olum majus, the so-called "Nasturtium" of gardens. 

 We select for study flowers which have only just opened, because 

 in older flowers the colour-bodies begin to be disorganised. 

 Let us first take surface sections from the upper side of the 

 sepals. The preparation can also be made with a fine pair of 

 forceps, if these are stuck pretty deeply into the tissue, and a 

 strip torn off. The preparation is laid in a drop of water, with 

 the epidermal side upwards. Examine it at once, because the 

 injurious action^of water on the colour-body 

 quickly makes itself felt. The margin of the 

 section will suffer immediately, so that cells 

 that are still unchanged should be selected 

 for more careful examination. The chro- 

 matophores are yellow, shading into orange- 

 red. They appear spindle-shaped, three or 

 four angled (Fig. 18), in forms which border 

 on, and in fact are, crystalline. When un- 

 changed they are homogeneous ; under the 

 influence of water they swell, become rounded 

 off, and vacuolate ; that is, hollows filled with 



water appear in their interior. They overlie 

 FIG. 18. From the up- . . , , , . ,, , ,, 



per side . of the calyx of m especial number the inner wall of the upper 



Tropwolum rtiajus The epidermal cells. The brown stripes on the 

 cell with the colour-bodies upper side of the sepals are due, as similar 



(chromatophores) adjacent . i_ i T < -i in 



to it ( x 540). sections snow, to bands ot epidermal cells 



which are filled with carmine-red cell-sap. 



These cells contain also yellow grains, which, however, the 

 coloured cell-sap makes almost invisible. In the red cells 



