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W. R. IVIMEY COOK.
the true Amœbæ, the motion being achieved by the extention and contrac- tion of blunt pseudopodia. Many remain in the cortical parenchyma, but some migrate into the cambial cells, this is possible because the amaba have the power of penetrating the cell walls and can therefore travel from cell to cell. In the cambium they increase their area of attack since in the subsequent meristematic activity of the cambium the parasite is distributed to many cells.
As a general rule, however, it is the medullary ray tissue rather than the xylem and phlam which is attacked.
cell.
After a while these myxamœbæ reach a cell supplied with plenty of food material, and they then proceed to grow at the expense of the host Sometimes, should two myxamabæ reach the same cell, they may coalesce, though not infrequently they appear to retain their individuality for a considerable time. It is for this reason that no suggestion is made that this coalescence represents a conjugation process. As the plasmodia increase in size the nuclei divide, and, as a rule, all the nuclei in the same plasmodium divide together.
The plasmodium, or multinucleated amœba, as it is termed by some writers, continues to grow, until it has devoured all the available food, or till it has filled the host cell. It then begins to develop spores.
b. Multiplication of the fungus by means of spores.
Their
up into a The protoplasm of the plasmodium becomes divided
Around number of fragments, each of which contains a single nucleus.
Each of these cach fragment of protoplasm a wall becomes laid down. masses constitutes a sporc. It is on the spatial distribution of these spores and on their arrangement relative to one another that the classification of themselves spores the genera is based. In Plasmodiophora Brassica the measure from 2μ—3o in diameter, though larger oval spores up to 4x6μ are occasionally found. In mature spores the wall is smooth, and has not any superficial markings. They are liberated only by the disorganisation of the host tissue, and there is no evidence that in this species the spores can germinate within the host tissues. Spores are essentially resting struc- tures, devised to tide the fungus over a period outside the host. vitality lasts several years, during which time they may become freely des seminated in the soil by wind, and by animal and human agency. spores, moreover, are able to withstand considerable desiccation without losing their vitality. When brought into a condition suitable for germina- tion, the spore-wall cracks open and there emerges a small swarm spore. is an oval or pyriform body, and possesses a single apical flagellum. measures from 3-3.5μ in length_and_1.5μ-2μ_in_breadth. flagellum is about equal in length to the swarm spore. Infection can only take place when the swarm spore germinates in fairly close association with the host plant. It has been shown that if the spores are sown more than The swarm spore five inches from the host plant no infection will occur. makes its way into the host tissue through the root hairs without any ex- ternal fusion, and up to this stage there has been no conjugation process. C. Infection of a fresh host plant by the fungus.
The
It
It
'The
The swarm spore penetrates through the cell wall of the root-hair, The Hong Kong Naturalist.
The Hong Kong Naturalist Supplement.
No. 1.
Plate 13, figure 1.
Plasmodiophora Brassicae, Spores x 320.
Plate 13, figure 2.
Plasmodiophora Diplantherur, Spores x 320.
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