138 A TEXT-BOOK OF BOTANY. 



ical basis of life," the chloroplastid has been spoken of as the 

 mill which supplies the world with its food, for it is by the 

 process of photosynthesis that the energy of the sun is converted 

 into vital energy, and starch and other products formed, which 

 become not only the source of food for the plant itself, but also 

 the source of the food-supply of the animals which feed upon 

 plants. In other words, horse-power is derived from the energy 

 of the sun which is stored in the starch grains of the chloroplastids. 



Chromoplastids. — In many cases, as in roots, like those of 

 carrot, or flowers and fruits, which are yellowish or orange- 

 colored, there is present a corresponding yellow pigment, and to 

 this class of pigments the name chromophyll may be applied. 

 Some of these pigments, as the carotin in carrot, have been iso- 

 lated in a crystalline condition (see Frontispiece, also Fig. 86). 



Chromoplastids usually contain, as first pointed out by Schim- 

 per and Meyer, protein substances in the form of crystal-like 

 bodies ; starch-grains may also be present. The chromoplastids 

 are very variable in shape and in other ways are markedly differ- 

 ent from the chloroplastids. They are more unstable than the 

 chloroplastids, and are formed in underground parts of the plant, 

 as in roots, as well as in parts exposed to the light, as in the flower. 

 Their formation frequently follows that of the chloroplastids, as 

 in the ripening of certain yellow fruits, such as apples, oranges, 

 persimmons, etc. 



The PLASTID PIGMENTS are distinguished from all other color- 

 substances in the plant by the fact that they are insoluble in water 

 and soluble in ether, chloroform, and similar solvents. In fact, 

 they are but little affected by the usual chemical reagents under 

 ordinary conditions. 



Apart from the difference in color, the yellow pigment (chro- 

 mophyll) is distinguished from the green (chlorophyll) by the 

 fact that the latter is said to contain nitrogen, and also by their 

 difference in behavior when examined spectroscopically, chloro- 

 phyll giving several distinct bands in the yellow and orange por- 

 tion of the spectrum, which are wanting in the spectrum of the 

 yellow principle. 



Cytology, or the science of cell formation and cell life. Dur- 

 ing recent years considerable attention has been given by botanists 



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