270 Journal of Agricultural Research voi.iv, Naj 



to stain. A few nuclei are shown in the infecting plasmodium in Plate 

 XXXIX, figure 4. A nuclear membrane, nucleolus, and, in some cases, 

 chromatin strands are to be seen. The nuclei stain much more readily 

 after they have been carried into the host cells. Nuclear divisions have 

 not been observed in the plasmodium before it enters the host cells. 



In the way which has already been described the invading plasmodium 

 infects a small pocket of tissue just beneath the epidermis. This varies 

 from I to 2 mm. in diameter and from one to six cells in thickness. Its 

 size determines the size of the powder}--scab sorus that is to result. If 

 this small island of infected cells could be removed, it would leave a 

 bowl-shaped cavity covered over by the epidermis. 



Shortly after the cells become infected, they begin to grow very rapidly. 

 Giant cells 5 or 10 times as large as the normal ones are soon to be seen. 

 Instead of growing equally in all directions, the infected cells elongate, 

 most of their growth being radially outward. This results in lifting the 

 epidermis and finally in breaking through it. Plates XLI and XLII 

 show the raised epidermis, while Plate XLIII, figure i, gives a good 

 idea of the appearance of the sorus after the epidermis has been rup- 

 tured. The torn edges turn back on all sides and give the appearance 

 so characteristic of powdery-scab lesions. The individual cells at first 

 become much enlarged. Their nuclei divide, sometimes mitotically, but 

 much more often, it is believed, by direct division. In those cases where 

 mitotic division of the nucleus occurs, a cell plate is formed and the cell 

 becomes divided in the usual way. When the nuclei divide directly, 

 the giant cells become multinucleate. Ultimately the giant cells are 

 all cut up into smaller cells. Usually not more than five or six cells are 

 produced by a single infected cell. Some of the giant cells and also some 

 of the vertical rows of small cells that have resulted from the division of 

 the large ones can be seen in Plates XLI and XLII. Plate XLI shows a 

 vertical section through the edge of a young sorus. The epidermis has 

 been slightly raised through the upward growth of infected cells. Just 

 beneath the epidermis can be seen the dark intercellular spaces which 

 were previously occupied by the infecting plasmodium. These spaces 

 are now filled with debris left behind when the plasmodium entered the 

 potato cells. The larger globules within the cells are the deeply stained 

 bodies which are so common in the plasmodium. None of these bodies 

 can be seen in uninfected cells. The smaller dark bodies are in most 

 cases the nuclei of the parasite, and in some of the cells they can be seen 

 clustered around the host nuclei. At this stage the host nuclei are 

 spherical, and each contains a red-stained nucleolus. In this section 

 most of the giant cells have been cut up into smaller ones, but two of 

 them are still present. It can be seen that most of their growth has 

 been radially outward. In Plate XLII can be seen a somewhat later 

 stage than was shown in Plate XLI. Each host nucleus is embedded in 

 a Plasmodium. The intercellular spaces beneath the epidermis, which 



