Von SCHRENK: TEACHING OF VEGETABLE PaTHoLocy 61 
air, and later on the protecting cork layer and the callus is fully 
brought out in that connection. I have found succulent stems and 
leaves very good for the study of wounds, particularly the leaves 
of the cauliflower. In this connection the wounds caused by in- 
sects can be studied, and the peculiar response which the living 
tissue makes to the disturbing cause. Plant lice abound every- 
where, and so do gall flies, so that there is never any trouble in 
getting living material for study. Following this one may observe 
the influence of living organisms in cells, for instance, the manner 
in which the various Chytridiaceae influence the leaf tissue. From 
these one may continue with the more complex reactions of fungi 
like Albugo, Exoascus, and Gymnosporangium. With all of these 
it is possible to keep the fungus, and the leaves attacked, under 
constant observation. With some it is not easy, for instance the 
Exoascus and Gymnosporangium ; these had best be observed out- 
of-doors in the spring ; with a little judicious planning it is feasible 
to postpone the study of these complex reactions until the latter 
Part of the course. Adbugo and Synchytrium can be grown on 
plants in greenhouses. Very different from the influence exerted 
by these fungi is that exerted by others like Botrytis, the Ure- 
dineae, and the Peronosporeae. It will be sufficient for me to re- 
fer to this simply, as the reasons why these should be included 
are obvious enough. 
If time permits, the changes in cell-groups near wounds and 
Spots attacked by fungi can be studied, 7. ¢., the formation of 
wound gums, the increased evolution of heat and the rapid trans- 
formation of plastic food bodies, the accumulation of starch, etc. 
A second line of work grows out of the question as to what 
Causes disease. This question involves most of the others men- 
tioned above, and can be made the starting point for all. Using 
the convenient division of disease caused by the action of either 
environmental factors of living organisms, one can begin by ex- 
Perimenting with unfavorable environmental conditions. In doing 
SO it is of course hardly necessary to say that great care must be 
mecd in devising experiments which will give definite results. To 
illustrate: if the influence of excessive heat is to be studied, one 
Must be sure that the light, moisture and gaseous conditions are 
te same. As a rule it will prove most satisfactory to work with 
