558 
Journal of Agricultural Research 
Vol. XIV, No. 13 
tion lies in the fact that most investigators have studied only the young 
rootlets. 
As is well known, badly diseased plants wilt during the warmer parts 
of the day, when transpiration is most rapid. For a certain time at least 
they recover from the wilting during the night and on cloudy or rainy 
days. Finally, however, the plants wilt beyond recovery, die, and dry 
up. The early stages of the disease cause more or less dwarfing, but, so 
far as the life of the plant is concerned, the critical period is reached when 
wilting begins. This is the symptom of approaching death. 
Most wilt diseases that do not result from an actual destruction of 
some organ of the plant are caused by vascular parasites. The invading 
organism gets into the vascular system and interferes with the passage 
of water from the roots up to* the leaves. Several writers have sug¬ 
gested that diseased plants have fewer lateral feeding roots than healthy 
plants. This is to a certain extent true, but it does not account for the 
wilting in all cases, since many plants wilt that are abundantly supplied 
with feeding roots. 
The writer has made a study of the vascular elements of infected 
roots and stems by means of stained serial sections. Small plasmodia 
have occasionally been found in the tracheids and also in the large 
vessels. But this is by no means common; it is, in fact, so rare that the 
possibility of these plasmodia having any appreciable effect on the 
functioning of the xylem elements is out of the question. Like other 
dicotyledonous plants, the cabbage during its secondary growth produces, 
on either side of the cambium, cells that sooner or later become differ¬ 
entiated into xylem and phloem. In the case of the cabbage plant, the 
differentiation takes place more quickly on the xylem than on the 
phloem side. The development of xylem tissue thus keeps pace with 
the increase in transpiration, due to the growth of leaves. When the 
young root or stem is attacked by the parasite in question the cambium 
quickly becomes infected, as has been seen. The plasmodia then pass 
into the layers of undifferentiated cells on either side of the cambium. 
These cells, instead of developing into vascular elements, as they do in 
healthy plants, are stimulated to abnormal growth and division. Even 
the noninfected cells surrounding those that contain the parasite are to a 
certain degree prevented from developing into vascular tissue. The 
differentiation of noninfected cells is, however, not entirely prevented, 
and a small amount of vascular tissue continues to be added to each 
bundle. The top of the diseased plant keeps on growing—not so fast as 
in the case of healthy plants—but much too fast for the atrophied develop¬ 
ment that takes place in the conducting system. In other words, the 
leaf surface outgrows the conducting system. The parasite may not 
interfere with the functioning of this system, but it prevents that enlarge¬ 
ment which would be necessary to meet the needs of the ever-increasing 
