QUEKETT MICROSCOPICAL CLUB. 343 



gi'anules or crystals. The crystals are carotin, so called because 

 first isolated from the root of the carrot. It occurs in some 

 flowers and in fruits, as the tomato. The most widespread pig- 

 ment in chromoplasts is yellow or yellow-orange. It is known as 

 xanthin. Nearly all yellow flowers owe their colour to it. 



The soluble pigments exist in solution in the cell-sap. The 

 most important and interesting is the series of anthocyanins, to 

 which are due most of the reds and blues, and combinations of 

 these in purples, violets, etc. They are almost universally dis- 

 tributed, and occur normally, or as the result of unusual condi- 

 tions. Anthocyanin iff often associated with damp- and shade- 

 loving plants, and is developed specially on the under surface of 

 leaves. The leaves of water-lilies and other aquatics are often 

 light green above and deep violet below. The variegated plants 

 of cultivation — Caladium, Dracaena, Coleus, Begonia, etc. — 

 were also referred to. The autumnal coloration of leaves is 

 due to anthocyanin. Low temperatures may induce its forma- 

 tion, and it occurs in Arctic plants and in many evergreen shrubs 

 and herbs during winter. The colour of copper beech and 

 similar varieties is due to its presence. Chemically, the antho- 

 cyanins are a class of substances, somewhat allied to sugars, 

 known as glucosides. They are soluble in water and generally 

 in alcohol, and may be obtained in the crystalline form. They 

 give a bright red colour when treated with acids, and generally 

 green with alkalies. A great deal of interesting work has been 

 done and much has been written on the relation of these sub- 

 stances to the process of photosynthesis, to the effect of tem- 

 perature, light, drought, presence of oxygen, excess of sugar in the 

 tissues and to other conditions and factors. Their physiological 

 significance has been much debated, two rival views having been 

 advocated : first, the " light screen " hypothesis ; secondly, the 

 "" heating" hypothesis of Stahl. A fair number of flowers owe 

 their colour to another series of soluble pigments, the xantheic 

 series, which are yellow, orange-yellow or red. They occur in 

 primrose, yellow rose, narcissus, eschscholtzia and others. The 

 colour of a flower is frequently a combination of effects, usually 

 of an anthocyanin in the epidermis overlying cells containing 

 chromoplasts ; where blue anthocyanin overlies yellow chromo- 

 plasts, green is formed, as at the base of the yellow petals in some 

 tulips. 



