AGRICULTURE: L. R. JONES 45 
2. How does this factor of disease resistance behave in inheritance? Is 
it surely transmissible and does it 'Mendelize?' 
3. Is the quality of resistance or susceptibility influenced by environ- 
mental factors, if so, how? Related to this is the question as to whether such 
resistance will persist when the strain selected in one locality, e.g., Wisconsin, 
is grown in a remote locality, e.g., New Jersey. 
4. Is the principle general? In other words, is there similar variation in 
individual susceptibility and resistance to this parasite among other varie- 
ties of cabbage than the one considered above? How general is the occur- 
rence of such differences with other host plants and other plant parasites? 
While wholly satisfying answers are not yet formulated for any of these 
questions, such evidence as has been secured may be summarized as follows. 
As bearing on the first question, W. H. Tisdale, working in our laboratory, 
has sought to learn what difference there is in the relations of the parasite 
to the resistant and the susceptible plants. Since the cabbage is a clumsy 
plant of slow development he used for his primary investigations the flax 
which is invaded by a closely similar parasitic Fusarium and which has well dif- 
ferentiated resistant and susceptible strains. Secondary comparative studies 
with the cabbage, while not yet complete, indicate a general likeness in be- 
havior. He has learned that the difference in resistance is not due to any 
superficial obstacle since the parasite, which may enter through the root 
hairs, penetrates these freely in the resistant as well as the susceptible plants. 
The difference is in the relations of the interior cells of the host and the para- 
site. In the suceptible plants the parasite penetrates directly to the vessels 
and then ramifies through them to the destruction of the host. In the re- 
sistant individuals, on the other hand, the invasion advances more slowly 
and before it reaches the vessels is checked and permanently walled off by the 
development of a corky layer. This reaction may be interpreted in various 
ways but the one we favor is that the resistant tissues have the ability to 
restrain the development of the parasite to a greater degree than do the 
susceptible and so give time for the protective cork formation. 
With reference to the second question, Tisdale 2 has also sought to follow the 
behavior of resistance as an inherited character again using the flax. Biffin, 
in England, working with wheat, concluded that resistance to the rust parasite 
(Puccinia) was inherited as a simple Mendelian character, being recessive. 
With flax the problem proves not so simple. Crossing highly susceptible 
with resistant strains and testing the progeny shows that resistance is clearly 
a transmissible character, but the hybrids are in general more or less inter- 
mediate in this respect with a tendency for resistance to be dominant. Much 
variation is found in lines originating from different parents. The indica- 
tions are that resistance is not a single character but a complex, dependent 
upon a number of heritable factors. 
As to the third question, environmental factors do have a marked influence 
in determining whether the Fusarium parasite will invade the cabbage. J. C. 
