. Botany of the living plant. Botany; Plants. 396 BOTANY OF THE LIVING PLANT It is along such lines that the explanation must be sought for the condition known as " epidemic," where by a sudden outburst a disease becomes prevalent. Examples have been seen in the Irish Potato Famine, the Coffee Disease of Ceylon, or the Lily Disease which in 1888 made the cultivation of Lilies in the Thames Valley a failure. In such cases the disease is not necessarily a new one. The novelty lies in the success of the invader. It appears to be due to a change of balance between attack and resistance.
. Botany of the living plant. Botany; Plants. 396 BOTANY OF THE LIVING PLANT It is along such lines that the explanation must be sought for the condition known as " epidemic," where by a sudden outburst a disease becomes prevalent. Examples have been seen in the Irish Potato Famine, the Coffee Disease of Ceylon, or the Lily Disease which in 1888 made the cultivation of Lilies in the Thames Valley a failure. In such cases the disease is not necessarily a new one. The novelty lies in the success of the invader. It appears to be due to a change of balance between attack and resistance. That balance may be affected either by physiological strengthening of the parasite, or by weakening of the host. Sometimes the same circumstances may affect both. In the Lily Disease and the Potato Disease a cold wet. Fig. 295. Portion of the root of a Crucifer malformed owing to the presence of Plasmodiophora. (After Woronin ; from Marshall Ward.) season, while it favours the fungus, produces a thin-walled, watery host, readily susceptible to attack. A similar epidemic of " damping off " by Pythium may at any time be induced by cultivation of Cress overcrowded, in moisture and heat (see p. 400). The effect of the parasitic invasion may be the death of the host, where vital parts are destroyed, as in attacks by Pythium, or Armillaria mellea (Fig. 292). But in many cases the attack is tolerated by the host, with only partial injury. It is often the leaf, or only certain tissues of the leaf, which are attacked, the result being a loss of efficiency by the host while the parasite gains access to the sources of supply. The host may even be stimulated to greater action, with the effect of swelling and extra divisions of its cells. The result may be various malformations, such as are seen in the familiar leaf-curl of Peaches or the swollen patches of Cluster-Cups (Fig. 337, p. 437)-. Please note that these images are extracted from scanned page images that may have been dig
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