Dorothy M. Cayley 465 



not tried, as Streak of Sweet Pea and bacterial disease of eating pea 

 are both so prevalent at this Institution that the results would be 

 wholly unreliable. 



So far Ps. seminum has not been found on any other host plant; 

 there is however no doubt that the organism occurs in the soil, as 

 healthy peas sown on infected land can develop the disease. Judging 

 from the fact that the organism can live a long time in artificial liquid 

 media, it can probably live for years in the soil. To give one example 

 of many records illustrating this capacity, a tube of sterile soil-extract 

 was infected with a pure culture of Ps. seminum, and it grew well 

 when plated after eight months and two days. Old platings from 

 other nutrient media gave similar results. 



Infection of the Host Plant. 



As mentioned above, the organism can only penetrate uninjured 

 tissues when they are very young. Plate VI, fig. 1 shows the penetra- 

 tion of the epidermal cells of the young embryo of a pea germinated 

 in sterile sand in a petri dish. It gains access into the tissues in the 

 rod stage chiefly through the radial walls of the epidermal layer, but 

 can also penetrate the young cuticle. The cell walls become swollen 

 and disorganised (Plate VI, figs. 1, 2, S.W.) and the organism forces 

 its way into the intercellular spaces and from there into the cells 

 themselves. The nucleus is often attacked, the cytoplasm destroyed, 

 and the cells dry out and rents occur in the tissues. These rents are 

 particularly marked in dry weather and occur chiefly at the nodes, 

 immediately below the base of the petiole of the leaf. Plate VI, fig. 2 

 shows the cells of the phloem parenchyma of a young stem, in which 

 two cell walls have been broken down ; and Plate VI, fig. 3 a cell of the 

 cotyledon of a badly diseased pea, after germination. This last figure 

 shows very clearly the penetration of the bacilli into the cells. 



The organism occurs in the phloem, cambium, medullary rays, and 

 occasionally in the pith, also in the parenchyma and vascular bundles 

 of pods, in the tissues of the funicle, the cotyledons, and the leaves. 

 It has no diastatic action on the pea starch. In Plate VI, fig. 3 a few 

 starch grains can be seen intact, but most have been dissolved out by 

 the chromacetic fixative. Unless some starch-dissolving fixative is 

 used it is not possible to cut good microtome sections of the cotyledon. 

 The bacilli have never been observed in the vessels. The occasional 

 wilting of diseased plants is probably due to some other cause, such as 



