360 GENERAL PLANT PATHOLOGY 



vapor. The individual chlorophyll grains may not attain their normal 

 size, remaining small. The formation of chlorophyll presupposes a cer- 

 tain temperature, the action of light, the presence of iron and certain 

 organic food materials. Low temperature may reduce chlorophyll for- 

 mation, as is seen in grain seedlings and bulbous outgrowths or with 

 yellowish color grown under a low temperature. Deficiency of light and 

 iron causes etiolation, more especially chlorosis, or icterus in the absence 

 of normal pigment due to the lack of iron, while in vines unable to 

 absorb iron chlorosis may take place with abundance of iron in the soil. 

 Sometimes it happens, on the other hand, following the attacks of an 

 insect that ripening lemons remain green-flecked. This condition 

 is due to arrested development of the chloroplasts, which normally 

 would be transformed to yellow chromatophores. 



Light also seems to influence the development of the red pigment, 

 anthocyanin, as is especially noticeable in varieties of Coleus, while 

 other parts, such as rhizomes, bulbs and roots, which remain under- 

 ground, are richly provided with anthocyanin. Chromogenic bacteria 

 may lose the power of producing pigment, as is illustrated by Micro- 

 coccus prodigiosus grown at the high temperature of 4o°C. A.F.W. 

 Schimper and other botanists have shown that the formation and dis- 

 tribution of crystals of calcium 'oxalate in plants is to a large extent 

 dependent on external factors. Shade leaves contain fewer crystals 

 than sun leaves and plants grown in moist air, or without light, are also 

 poor in these crystals. 



C. Tissue Differentiation. — The arrestment of tissue differentiation 

 can be illustrated in simple algae where the cells are united into colo- 

 nies. When the green alga, Scenedesmus caudatus, the end cells of 

 which have gelatinous horns, is subjected to abnormal life conditions 

 the horns do not form. In the consideration of tissues of multicel- 

 lular growths it may be said that there is no organ in which homo- 

 plasia may not appear. Examples have been found in the hepatic and 

 true mosses. 



The best illustrations of the developmental arrest of tissues are 

 found among the flowering plants, where as one case the guard cells of 

 the stomata may be arrested by a lowered transpiration and weak illumi- 

 nation. Stapf in his experiments with the potato, Solanum tuberosum, 

 showed that under normal conditions there was one stoma for every 

 forty-six epidermal cells, and in specimens matured by him in gaslight, 



