146 DISEASES OF GLASSHOUSE PLANTS 
than normal leaves. He confirmed the previous findings 
concerning the abnormality of the palisade cells, and 
noted the degeneration of cell contents. Slightly affected 
cells differed from healthy ones in that the chloroplasts 
were a paler green than usual. Increased effects were 
accompanied by a reduction in the number and in the 
depth of colour of the chloroplasts, and in severe cases 
they coalesced to form irregular green masses. Finally, 
the latter broke up into small hyaline bodies, at which 
stage the cell contents were observed to move very 
rapidly, and he concluded that the rapidity of movement 
of the granules was greater than could be accounted for 
by ordinary protoplasmic streaming. In the dark green 
areas the cells were generally larger than usual and con- 
tained more chloroplasts, which were of a darker green 
than is normally the case. 
The Infectious Nature of the Disease 
In certain plants, more especially the tomato, a 
mottling or variegation of the leaves frequently occurs, 
and this apparently results from malnutrition induced by 
unsuitable soil conditions. This state, which is generally 
known as “ chlorosis,’ may be confused with true mosaic 
if the basis of comparison is solely a macroscopic one, but 
the two disorders differ materially in one important 
respect, namely, the infectiousness of the plant juices. 
The investigations of ITwanowski in 1892 led to the dis- 
covery that the extracted juices of a tobacco plant 
infected with mosaic disease are capable of infecting a 
healthy plant when pricked into the tissues, even after 
passage through a Chamberland filter. His results were 
confirmed by Beijerinck (11), in 1898. 
This fact has now come to be recognized as a critical 
character of mosaic disease, and distinguishes it from 
non-infectious disorders of the “‘ chlorosis”’ type. True 
mosaic disease, therefore, is of an infectious nature, and 
is readily spread from plant to plant by natural and 
