2 BIXLETIX 1038, U. S. DEPAETMEXT OF AGRICULTURE. 



total suppression of chlorophyll, but since a yellowing or bleaching 

 of normally green parts maLj result from a wide variety of causes 

 chlorosis in itself is a symptom rather than a disease. There are, 

 for example, the chloroses of etiolation and of the normal autumnal 

 ripening of leaves. Moreover, a yellowing of chlorophyll frequently 

 follows upon some types of insect injury, such as that of root aphids 

 on the peach and the grape, and it is a constant accompaniment of 

 certain diseases caused by parasitic bacteria and fungi, such as bac- 

 terial black-rot of cabbage and Fusarium wilts of cabbage and potato. 

 Again there are the more or less general chloroses due to unfavorable 

 soil or climatic conditions, and finally those infectious chloroses of 

 obscure origin which present a fairly regular sequence of pathologi- 

 cal signs, including fundamental derangements in both metabolism 

 and morphogenesis. 



Some of these chloroses are true diseases in the restricted sense. 

 Others are not diseases except under a broad application of the 

 term, and certain forms of chlorophjdl restriction are clearly not 

 diseases at all. 



There are, however, two fairly well marked types of chlorophyll 

 disturbance which are usually included under the chlorotic group of 

 plant diseases. These are (1) the infectious chloroses which are 

 communicable through expressed plant juices or through those juices 

 as directly transmitted within the living plant tissues, and (2) the 

 noninfectious chloroses due to unfavorable soil or atmospheric con- 

 ditions. 



The present study of jDecan rosette deals with the histology and 

 cytology together with the sequence of gi'oss symptoms of the dis- 

 ease. In order to place the results of this study in proper relation 

 to other diseases of this type it is necessary to review briefly some of 

 the work of other investigators. 



CHLOROSES DUE TO SOIL OR ATMOSPHERIC CONDITIONS. 



With respect to those chlorophyll changes due to physical or 

 chemical conditions of soil or atmosphere it is difficult to say just at 

 what point the normal state ends and chlorosis begins. 



Certain plants prefer an acid condition of the soil, others tolerate 

 it, others are restricted for their optimum development to neutral 

 or alkaline situations. Nevertheless, neither macroscopic nor micro- 

 scopic examination of such a plant as field sorrel {Rinnex acetosella 

 Linn.), for example, would give a clue to its acid-soil toleration. In 

 resj)onse to certain environmental changes, however, Transeau (78) 

 has shown that this species does develop anatomical changes. In 

 moist situations, with soil and air temperature approximately iden- 

 tical, the leaves of this species are relatively large, with a loose 



