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are shown. Note particularly the pink-flowered form of N. Tabacum L. 
and N. paniculata, if available. oBSERVE:— ; 
8. The mottled or blotched appearance of the corollas of N. 
a Note the variations and compare with the flowers of healthy 
plants. 
9. That some of the flowers show a depauperate growth of 
the corolla; the stamens and stigma extrude. 
10. That the buds may be depauperate and variously mis- 
shapen. 
led. 
Make prawincs of flowers and buds, both healthy and diseased, to 
show the various symptoms. Indicate all points, which cannot be shown 
in drawings, by notes in the margin. 
On tomato leaves. Examine the tomato plant and oBSERVE:— 
12. The mottling of the leaves; their texture and flexibility; 
their size and shape. DRaw and note characters in the labeling. 
On petunia leaves. OBSERVE:— 
13. Their color, size and flexibility. praw and label fully. 
The symptoms on other hosts are similar to these. 
11. That the corolla-tubes of N. paniculata are bent and crump- 
‘ ETIOLOGY 
The cause of this disease is unknown but there are various theories 
formulated to explain it. Some investigators (Woods, Chapman and others) 
believe that enzymes play an important, if not the primary, part in causing 
it; oxidase and peroxidase.are specifically named. It is said that rough 
handling of the plants, such as breaking the roots in transplanting or 
severe pruning of the tops, will cause the disease. Disturbance of the 
equilibrium between root-absorption and leaf-transpiration is also held to 
be responsible for mosaic. 
A much more plausible theory is that recently offered (by Allard and 
others). The disease is attributed to a filterable virus. It is held that 
this virus consists of an ultra-microscopic organism, bacterial or proto- 
zoan in nature. The extreme minuteness or plasticity of the organism 
probably allows it to pass through the pores of the Chamberland filter. 
Such phenomena are known in other cases. 
Crush small pieces of healthy and diseased tobacco leaves in small 
drops of water, cover, and examine each with the high-power. OBSERVE :— 
14. The absence of any evidence of an organism in the diseased 
tissues. 
Pathological Histology. The effects on the structure of the leaf are 
very pronounced. Make thin cross-sections through diseased and healthy 
tobacco leaves, or if available study prepared slides. OBSERVE:— 
15. The arrangement and contents of the cells of the palisade- 
layers, spongy parenchyma and epidermal tissues of the healthy leaves. 
DRAW in detail a section across the leaf. 
16. The corresponding cells in the green and yellow area of the 
diseased leaf. Note the suppression of the palisade-tissue in the yellow 
areas. The relative thickness of the green and the yellow areas. DRAW 
a section across a diseased leaf at the same enlargement as for the section 
across the healthy leaf. 
