Abstract1. Root galls caused by nematodes (Heterodera marioni) originate as a result of attacks on meristematic tissues of very young roots. 2. The parasite may locate in the cambium or cortex or penetrate in the medullary rays. 3. There is no evidence that the parasite can attack fully formed fibrous and tracheary tissues or that it can stimulate their growth, but it can push them out of place. 4. The cambium, cortex and medullary rays are stimulated to excessive cell division and tracheary tissue is formed in the cortex. 5. The cells next to the parasites during the period of gall formation are very small as a result of excessive cell division while the more remote cells are much larger.
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