64 



INTRODUCTION TO CYTOLOGY 



The most important of all plastid pigments is chlorophyll, because of 

 its peculiar relation to the elaboration of carbohydrates by chloroplasts. 

 Chlorophyll ordinarily develops in the plastid only in the presence of light; 

 apparent exceptions are found in certain woods and the embryo and endo- 

 sperm of certain seeds. Other conditions necessary for the development 

 of chlorophyll are a favorable temperature and the presence of iron, 

 oxygen, and certain carbohydrates. Most young seedlings grown in the 

 absence of light show a pale yellowish color. When such 

 "etiolated" plants are placed in the light, the plastids 

 become green, apparently through an alteration of "chloro- 

 phyllogen" to chlorophyll. There is a close chemical 

 relationship between chlorophyll and the haemoglobin of 

 animal blood. 



With chlorophyll are usually associated one or both of 

 the yellow carotinoid pigments carotin and xanthophyll. 

 The exact mutual relation of the several pigments in the 

 chloroplast is uncertain, some investigators thinking it 

 probable that a single green compound decomposes readily 

 into "chlorophyll a," "chlorophyll 6," and other products 

 when subjected to analysis (Lubimenko). In the algae 

 Cell of Spiro- Other pigments, notably brown phycophsein and fuco- 

 gyra, with spiral xanthin, red phycoerythrin, and blue phycocyanin. 

 The nucleus is occur in addition to chlorophyll, though in the case of the 

 suspended in the blue-green algsB there is some question concerning the 

 by a series of limitation of pigments to plastids (Chapter XIII). Red 

 cytoplasmic and yellow pigments may develop either in the leucoplasts 

 meet the pe- directly or in chloroplasts as the chlorophyll disintegrates 

 rip herai cyto- (Meyer). In tomatoes the red lycopin, an isomer of 

 the pyrenotds! carotin, appears in the chloroplasts as the fruit ripens. 

 {After Couch, The colors of flower petals are in some cases due to plastid 



1 Q*^ 1 /i ^ 



pigments. In Nasturtium, for example, the yellow color 

 is in chromoplasts, but red pigment in the same flower is held in solu- 

 tion in the cell sap.* 



Photosynthesis. — Of all chromoplasts the chloroplasts stand first in 

 importance, for by virtue of their chlorophyll they are able, in the pres- 

 ence of light, to recombine the elements of carbon dioxide and water in 

 such a way as to form certain carbohydrates, oxygen being given off as a 

 by-product. In this process the energy of sunlight is fixed, while in 

 respiration this energy is again released for use by the organism. Repre- 



^ For accounts of plastid pigments, see the works of Palladin (1923), Onslow (1923), 

 R. W. Thatcher (1921), Beauverie (1919), Willstatter and Stoll (1913), Jorgensen and 

 Stiles (1917), Haas and Hill (1928), Rigg (1924), Oltmanns (1923), Palmer (1922), 

 Fulton (1922), Lloyd (1924), Mobius (1927), and Gortner (1929). The distribution 

 of carotin is discussed in an earlier paper by Tammes (1900). 



