60 BOTANY 



rophyll there are two other pigments present, a reddish one, Carotin, 

 and a yellow one, Xanthophyll. These, like the chlorophyll, seem 

 to be dissolved in an oily substance, which is extracted by alcohol 

 and other solvents. If an alcoholic solution of chlorophyll is exam- 

 ined, it shows a strong fluorescence, appearing reddish by reflected 

 light. If the green alcoholic solution is shaken up with benzole, on 

 settling the latter will dissolve out the yellow xanthophyll, leaving 

 the chlorophyll in the alcohol. 



The Chromoplasts differ from the chloroplasts in their red or yellow 

 color. They give the color to many red and yellow flowers like the 

 Nasturtium and Marigold, and the yellow and orange fruits, such 

 as Rose-hips, Mountain-ash, Pumpkin, Peppers, Squash, etc. They 

 may differ but little in form from the ordinary chloroplasts, from 

 which they are often directly derived, or they may arise from small 

 indifferentiated chromatophores. Sometimes they are of very irregu- 

 lar forms (Fig. 31, D), owing to the crystallization of the pigment. 

 This is either carotin or xanthophyll, the relative abundance of 

 which renders the chromoplast either red or yellow. The yellow 

 color of leaves deprived of light is due to their failure to develop 

 chlorophyll, thus rendering visible the yellow pigment, xantho- 

 phyll (etiolin), which is hidden by the chlorophyll in the normal 

 chloroplast. 



Leucoplasts. If we make a thin longitudinal section of an herba- 

 ceous stem, it may usually be seen that the chloroplasts of the 

 outer cells are replaced by similar but colorless bodies in the inner 

 cells where the light is more or less cut off by the overlying tissues. 

 Every gradation between the true chloroplasts and these colorless 

 leucoplasts may often be found. Leucoplasts which occur in roots, 

 or other subterranean parts, may, when exposed to the light, develop 

 into normal chloroplasts. This is clearly seen in the outer tissues 

 of potato-tubers, which soon turn green when exposed to light. 



Starch-formers. One important group of the leucoplasts are the 

 starch-formers (Fig. 31), which occur in tissues where reserve-starch 

 is being manufactured. The starch-grains arise within the leuco- 

 plast, just as they do in the chloroplasts when exposed to light, but 

 the formation of the starch-grains by the leucoplasts is quite inde- 

 pendent of light, and the materials of which the reserve-starch is 

 composed are derived from the starch manufactured in the chloro- 

 plasts under the influence of light. 



The Cell-wall 



Unlike the nucleus and plastids, the cell-wall is not a permanent 

 organ of the cell, but may be renewed from time to time. The cel- 

 lulose found in the walls of most young plant-cells is replaced in 



