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on the contrary, partly oxplaiiiwl liy tlio circuinsfaiice that it must 

 first be bound to the protoplasm of the living' cell itself, and so 

 carried on into tlie reproduction. In any case, two enigmas seem 

 by this fact to a certain degree to be reduced to one '). 



If the ground in which the tobacco-plants grow is infected with the 

 virus, one sees, after some time, the disease appear in the terminal bud. 

 The time of the incubation varies very much and depends on the 

 size of the plants. In smaller plants I remarked the first symptoms 

 of the disease in the new-formed leaves of the terminal bud two 

 weeks after the infection ; in bigger ones after three to six weeks. 

 Root and stem are in tliis case obliged to convey the virus often 

 to considerable distances. Yarious observations prove that this con- 

 veyance goes, by way of exception, along the xylem, which conducts 

 the water; usually, however, it seems to follow the so-called descending 

 sap, and then it probably goes along the phloem. 



That the first mentioned way may be followed, must be concluded 

 from the order of succession in which the symptoms of the disease 

 appear when a great quantity of the contagium is introduced into 

 the stem, for in this case those parts of the young leaves get first 

 diseased which are exposed to the strongest evaporation, such as 

 the tops and margins which reach freely out of the bud. The con- 

 veyance of the virus along the phloem may be concluded from the 

 following observation. 



If one infects the middle-ribs of full-grown leaves, or of leaves 

 whose cells are in a state of elongation but no more of cell-division, 

 the leaves themselves continue healthy, but the virus turns back to 

 the stem, thence, in the usual way, to infect the meristems of the 

 buds and the youngest leaf-rudiments. This return of the virus from 

 the leaf down to the stem must undoubtedly be by the way of the 

 descending sap, that is along the phloem. 



') Perluips, basins^ on these considerations, one may fliink of tlie possibility that 

 enzymes, in a manner agreeing with tbat of the contaginm flnidum, are reproiiiiced 

 in tbe cells, and migbt tbns, to a certain extent, be considered as independently 

 existing. Concerning tbis point I wisb to observe tbe following. Pressed yeast, culti- 

 vated in nutriment containing diastase, takes from it a notable quantity of diastase 

 wliich it is difficult completely to remove from it by washing. If, however, tbis yeast 

 continues growing in a medium free from diastase, then the diastase soon totally 

 disappears. That this might be attributed to incongruence between tbe protoplasm ot 

 the yeast-cell and the diastase-molecule, so that a persistent union might possibly be 

 effected by other microbes or by means of the tissue-cells of higher organisms, is not 

 probable, the yeftst-cell being not absolutely free from diastatic substances, contiiining, 

 for instance, some glucase. For the moment, therefore, I nuist consider such a con- 

 (dusiou as unavaihible. 



