2 S. G. WILDMAN 



of the monumental discovery that plant viruses are nucleoproteins. Except 

 for the war years, the rate of published paper production has continued to 

 rise, with no abatement in sight. 



The welter of plant virus literature can be classified into three main com- 

 partments. The largest compartment contains a wealth of descriptive infor- 

 mation on the great variety of diseases which viruses produce in plants, the 

 mode of transfer of viruses from plant to plant, kinds of symptoms induced 

 in the hosts, etc. A second compartment contains a truly impressive know- 

 ledge of the chemical composition and physical construction of plant viruses 

 as they are observed in an extracellular condition. The third compartment 

 contains our current, but insufficient knowledge of the mechanisms involved 

 when a virus interacts with a plant cell to set in motion the phenomenon of 

 virus self-rephcation. Much of the experimental information has come from 

 the study of tobacco mosaic virus (TMV). Compared to the others, the third 

 compartment is just beginning to fill. Enough is known, however, to trace the 

 rough outlines of virus development within living plant cells, and sketching 

 this picture will be the objective of this chapter. 



Since it has not yet been possible to separate plant cells in a way that 

 would permit investigation of virus activity within single cells, as in the case 

 of bacteriophage and animal viruses, all of our information rests on the 

 analysis of virus infection of plant organs. Since the leaf has served as the 

 principal organ for experimentation, it is approj)riate to commence this dis- 

 cussion by considering the organization of a typical leaf before proceeding 

 toward analyzing the effect of virus infection on leaf cells. 



General Features of the Host-Virus System 

 A. Organization of Plant Leaves 



Seen in cross section, a leaf is a multilayered structure. Both the upper and 

 lower surfaces of the leaf are covered by a single layer of epidermal cells. 

 Directly beneath the upper epidermis layer is found another single layer of 

 closely packed, columnar cells known as the palisade parenchyma layer. 

 These elongated cells have their greatest dimension miming at right angles 

 to the epidermal layer. Beneath the palisade layer are found a few layers of 

 loosely packed cells kno-^Ti as the spongy mesophyll. These cells are more or 

 less circular in outline and, like the palisade layer, the spongy cells are 

 packed with green chloroplasts. The lowermost layer of the spongy meso- 

 phyll is bounded by another single layer of cells, the lower epidermis. The 

 two epidermal layers are virtually indistinguishable from each other, but are 

 sharply set apart from the palisade and spongy layers by the fact that they 

 contain few chloroplasts and appear almost colorless. 



