PROCESS OF INFECTION AND VIRUS SYNTHESIS 3 



The leaf is further characterized by the presence of small pores, or stomata, 

 appearing in the epidermal layers. The stomata permit the entrance and 

 exit of gaseous materials from the palisade and spongy layers of the leaf. 

 Furthermore, in the two species of tobacco extensively used in TMV studies, 

 the upper and lower surface of the leaves are densely covered by leaf hairs. 

 Leaf hairs are extensions of epidennal cells, and in the case of Nicotiana 

 glutinosa and N. tabacum, are of four different kinds, which vary from being 

 composed of only 1 or 2 Uvmg cells to as many as 6 to 7 cells, arranged in a 

 row and capped by a group of small cells at the tip of which there is secreted 

 a sticky substance that adheres to fingers when they come in contact with 

 tobacco plants. 



Looking down on the upper surface of a tobacco leaf, one would expect to 

 see about 10* epidermal cells per mm.^ Roughly one-third of these cells 

 would have leaf hairs protruding from their surface. Each epidermal 

 cell, in turn, covers between 4 and 5 palisade cells. Coursing through the 

 interior of the leaf are still other types of cells which comprise the 

 veins and vein endings. The average tobacco leaf contains about 10^ 

 cells. 



A conspicuous feature which sets the plant cell apart from the animal and 

 bacterial cell is the presence of a cellulose cell wall. From all available evi- 

 dence, the cellulose wall is an impenetrable barrier to the passage of even the 

 smaDest protein molecules (Chibnall, 1939; Wildman and Cohen, 1955). How- 

 ever, where leaf cells abut on each other, the ceUulose wall is pierced by 

 tiny openings through which strands of protoplasm, known as plasmades- 

 meta, connect the protoplasm of one cell to another. Thus, we can look at 

 the leaf, on the one hand, as being completely surrounded by a total ceUulose 

 barrier, while, on the other hand, the protoplasm can be viewed as a con- 

 tinuum. Consequently, once passage through the cellulose barrier is gamed 

 by a foreign object such as a virus, the gate is opened for its potential passage 

 through every cell of the leaf. 



Another feature that contrasts a leaf with animal organs and bacterial 

 populations is the fact that most of the visible growth of a leaf is by the 

 process of cellular elongation and expansion. Origin by mitosis of the new 

 cells which will develop into a leaf occurs very early in development and 

 mainly before the leaf becomes visible to the naked eye. 



This growth behavior results in the peripheral distribution of the proto- 

 plasm in the leaf cells. The thin layer of protoplasm enclosed in the cell wall 

 in a fully grown cell may be less than 10 microns thick, and surrounds a 

 central volume known as the vacuole, the latter accounting for 80-90 % of 

 the total cell volume. As nearly as can be told, the vacuole is devoid of 

 protein and visible structure, and is thought not to participate in virus 

 reproduction. 



