THE METHOD OF MOVEMENT 211 



approximately the same rate whether it was introduced 

 into older basal leaves or young apical leaves or into young 

 axillary shoots. To vary the probable direction of food 

 movement he placed entire plants in darkness for 48 hr., 

 inoculated an older leaf, and then placed the plant in the 

 light keeping the inoculated leaf in darkness. The virus 

 spread as rapidly and as completely from this darkened 

 and starved leaf as from a similar mature leaf exposed to 

 Ught or from a very young leaf near the apex. As he says, 

 this movement of virus from a darkened leaf, that is prob- 

 ably receiving sugars from the tissues to which the virus 

 is moving, rather clearly indicates a movement of virus 

 independent of the movement of carbohydrate. It is 

 possible, however, that even this starved and darkened 

 leaf was losing its contents to other parts of the plant and 

 was receiving nothing, for leaves that are about to die or 

 abscise still lose their stored foods to other tissues (see 

 Par. 3, Sec. 30), and Schumacher (1931) has demonstrated 

 the removal of nitrogen from such darkened leaves. The 

 spread from a darkened axillary shoot of tomato, however, 

 would seem to demonstrate a movement of virus out of a 

 tissue that was almost certainly receiving foods unless in 

 inoculating the axillary shoots Caldwell inoculated leaves 

 that perhaps receive no food when darkened. His descrip- 

 tion of these experiments is not sufficiently detailed to 

 make this point clear. 



Holmes (1932) reports experiments on the movements of 

 the tobacco virus in Nicotiana tabacum which rather clearly 

 demonstrate that the virus follows the same path as that 

 taken by carbohydrates. The virus leaves a leaf, however, 

 at about the same rate whether the leaf is supplying 

 carbohydrates to other parts of the plants or has been 

 darkened during the experiment. This is comparable to 

 the observation of Schumacher (1933) that transport of 

 fluorescein from a leaf is independent of carbohydrate or 

 nitrogen transport. If all other leaves were removed and 

 the inoculated leaf was exposed to light, however, Holmes 

 found a somewhat earlier appearance of virus symptoms 



