6 oo 
Journal of Agricultural Research 
Vol. XXVI, No. 12 
The same “hypertrophy” of the nucleus was noted in Puccinia graminis 
on oats. Eriksson believed that these enlarged nuclei represent com¬ 
bined host and fungous protoplasm. Magnus (23) finds striking enlarge¬ 
ment of nuclei in the orchid tissues infected by an endotrophic mycorrhiza. 
Ritter, in his studies of traumataxis of nuclei (31 ), found that in any 
cell showing the maximal reaction (i. e., where the cytoplasm and nucleus 
banked up on the side of the cell nearest the wound) the nucleus was 
above normal in size. Careful measurements were made of nuclei in a 
considerable area about the wound. The increase is greatest near the 
wound and is gradually less marked as the distance from it increases. 
Nestler (27) also noted the increase in nuclear size in tissues adjoining 
wounds. In an extreme case the diameter increased from 10 \x to 24.6 11. 
In the later stages of the reaction the size may return to normal. In 
some tissues this reaction is associated with nuclear and cell divisions, a 
renewal of growth that helps to heal the wound. He connects the increase 
in nuclear size with the altered metabolic relations (Emahrungsver- 
haltnisse). 
The expansion of the nucleus, then, is another common reaction which 
may occur under a variety of circumstances, and may indicate renewed 
or increased activity. 
It would be impossible to say with any certainty how many and what 
forces are at play in the production of the changes observed in host 
nuclei and plastids in the infected areas. Not all types of reaction to the 
rust have been studied. Any hypothesis at present must be tentative 
and incomplete. 
According to current belief, the nucleus is intimately concerned with 
the metabolic activities of the cell, and it may be that the increase in 
the size of the nucleus can be taken as an index of its heightened activity 
and that the condition of the plastids is more or less closely correlated 
with that of the nucleus. The fungus makes heavy demands for food (in 
susceptible hosts at least) upon the tissue it invades. Just how the extra 
burden imposed by the fungus upon a cell induces the production of 
additional food by that cell is hard to say. At any rate, at the center of 
the infection in Baart, where the demands of the fungus are greatest and 
where also the nuclei enlarge most rapidly, the reduction in size of the 
plastids is soon checked and a balance of forces is struck and maintained 
until the nuclei collapse. Farther out, at the margin of the infection and 
the area just beyond it, the nuclei respond more slowly, and there the 
plastids show extreme reduction (cf. 10a and 10b in Table I). A little 
later, however, the nuclei of these same outer areas expand (just before 
their collapse) and we find correspondingly a partial recovery in size of 
plastids in these areas. Tallying with this are the facts already mentioned, 
that between two uredinia, in areas affected by both fungi, the nuclei 
enlarge sooner, and here, too, we do not find the extreme reduction in 
plastid size. 
In Kanred nuclear expansion is proportionally quite as great as in 
Baart, and the response comes sooner in the outlying portions of the 
infection. Corresponding to this, the reduction in plastid size is more 
uniform throughout the infection, and, so far as observed, is not extreme. 
In Mindum the relationships are different. The haustoria are sup¬ 
posedly secreting the same substance or substances into the host cells 
that they did in the other cases, but Mindum protoplasm differs in some 
fashion in the nature, the concentration, or the organization of its chem¬ 
icals, and instead of being stimulated to greater activity, it is killed 
