8o 



ESAU 



of viruses limited to certain tissues of the host in distinction to those not so 

 limited. 



The typical abnormalities of mosaic-diseased plants — the disease is called 

 mosaic because of the mosaic combination of yellow and green patches on 

 leaves (fig. 1) — the underdevelopment and the breakdown of chloroplasts 



GREEN 



Fig. 1-S. Effect of mosaic disease upon the sugar-beet leaf. — Fig. 1. Mosaic pat- 

 tern on leaf. Green areas are shaded, yellow areas are left blank. — Fig. 2. Mesophyll 

 from a green area. It shows a loose arrangement of cells and numerous chloroplasts. — 

 Fig. 3. Mesophyll from a yellow area. It shows compact arrangement of cells like 

 a young leaf. This underdevelopment is one of the expressions of hyperplasia. The 

 chloroplasts are few. The deficiency in chloroplasts makes the tissue appear yellow. — 

 Fig. 4. Cell from green mesophyll with numerous chloroplasts. — Fig. 5. Cell from 

 yellow mesophyll. The chloroplasts have become partly or completely disorganized. 



are detected in chloroplast-containing tissues, mainly the mesophyll of the 

 leaves (fig. 2-5; see also Esau, 1944). Commonly, mosaics are associated also 

 with the development of peculiar structures in the protoplasts of the affected 

 cells, the so-called inclusion bodies (fig. 6). These may be found in all kinds 

 of living tissues of the host, including the tissues specialized for long-distance 

 conduction of food and water, that is, the phloem and the xylem, respectively. 

 In other words, the distribution of the internal symptoms clearly indicates 

 that the effect of a mosaic virus is not localized in any specific tissue. Tests 



