54« 
Journal of Agricultural Research 
Vol. XIV* No. xa 
the disease. While Favorski (4) expressed the opinion that infection 
takes place through ordinary epidermal cells, he, too, believes that only 
young roots are susceptible. This mistaken notion has been a great 
hindrance to a correct understanding of the disease. All previous at¬ 
tempts to study early stages of infection have been carried out with 
young rootlets. The infection is undoubtedly very rapid in such young 
organs. The cell walls are thin, and the parasite has to pass through only 
a few layers of cells before it reaches the central portions of the root. In 
older tissues the penetration is more difficult; the organism must pass 
through many layers of cells, and the study of its spread from tissue to 
tissue is much easier to accomplish. For this reason the writer has used 
rather old cabbage stems in his study of tissue invasion. 
The method has been to pull the earth away from the stems of potted 
plants, place a bit of inoculum on one side of each stem, and then put the 
earth back in place. Portions of the stems of these plants were then 
fixed in Flemming's stronger solution at intervals of one day. This 
fixing began one day after inoculation and continued for three weeks. 
At the end of this period infection was evident in the plants first inocu¬ 
lated, for the swellings had reached a considerable size. These stems were 
embedded in paraffin, sectioned, and stained. None of the cells of any 
of the stems studied became infected during the first eight days after the 
inoculum was placed on them. Some of the stems showed a few infected 
cells on the ninth and tenth days. These cells were in the outermost 
portions of the secondary cortex. No abnormal growth of infected cells 
or of cells surrounding them could be observed. On the eleventh day a 
very small swelling was seen on most of the stems, and somewhat deeper 
layers of cells showed infection. In a number of cases those swellings 
were so slight that they could not be seen with the naked eye, and it was 
not known that the overgrowth had started until after stained sections 
were studied under the microscope. Plate 64, B, shows a section through 
a swelling on a stem that was fixed 11 days after inoculation. Only the 
outer layers of the secondary cortex are infected. The parasite has al¬ 
ready stimulated these layers, and they are beginning to show abnormal 
growth. The dark specks that may be seen in the tissue of the protu¬ 
berance are the young plasmodia. They are as yet very small, usually 
showing not more than half a dozen nuclei and little of the oil so charac¬ 
teristically present during the period of their vegetative growth. These 
small plasmodia gradually increase in size as we pass to later stages of 
infection. This is well shown in the illustrations. It is interesting to 
note that the nuclei of the host cells and also the nuclei of the cells im¬ 
mediately surrounding the region of infection are more than twice their 
normal size. Although the inner cortical tissues are still free of infec¬ 
tion, the nuclei of the cells in the cambial region beneath the small plug 
of infected tissue are much larger than normal cambium nuclei. The 
