西瓜の蔓割病(麥凋病)に關する病理學的研究。 : IV. 被害植物の病理解剖
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The present paper reports the results on the anatomical studies of the affected grown plants, with special reference to their rootlets, main roots and stems. F.asarium niveum which has entered the host plant at the top of the rootlet, soon invades the vascular tissue, and finally fills up the disintegrated stele with mycelia (Fig. r, A B.). When the pro j ection of the secondary roots from the diseased roots takes place, majority of them may be affected with the casual fungus at their steles, before their root tops pass through the cortex parenchyma of the mother roots (Fig. 2). In the case of these pathological changes happened in the majority of the rootlets' of the host plant, it will be certain that the latter may wilt, when soil water content decreases to a certain point. When the grown plants are affe c t ed at some of their rootlets, none of them may become diseased, however the inoculated parts may be strongly affected. It is suggested that the root pressure supported by many other healthy rootlets may prevent the ascending stream from the diseased rootlets which scarecely have their own root pressure, and that the premature suberization of the exo- and endodermis and the tyloses formed within the affected vessels may hinder, for a while, the further invasion of the pathogen into the upper healthy reots. In main roots and stems of the affected grown plants, it is observed that the cortex parenchyma and the leptome disintegrate gradually and wither to flattened shapes with brown colour. The cells of the medullary ray and of pith parenchyma together with that of cambium at first become hypertrophied then disintegrate to form large hollows, in which yellowish resinlike substances are often observed. These pathological changes of these tissues are as a rule preceeded by that of hadrome. The affected hadrome shows no morphological change. The substances found in the vessels are such as, mycelia, tyloses (Fig. 3) and resinlike substances. None of these substances, however, are found to be the direct cause of the wilting. The tyloses, notwithstanding SLEETH's'' opinion, are observed to be the products from the living elements around the useless vessels, which have been damaged at some other places of the water conductive systems. The causal fungus which has intruded into the root tep and has invaded into the stele, soon gains its position in the vessel far from the infected portion by the aid of the migration of microconidia.y' The mycelia of the new colony thus formed within the upper healthy vessels, may invade the living tissues around the vessels, intruding from the thinner membranes of the pitted vessels. Thus the bodered tissues of the vessels are affected successively and at last they may become hollow, being distintegrated their cell contents (Fig. 7, B ; 8, B.c). In the case of watermelon, looking upon the construction of its water conductive tissues, it is clear that these vessels with hollow surrounding tissues may be impossible to act their function. If such pathologica l changes may be the case not only with a few vessels but also with the majority in a greater part of the vascular bundles, connecting with each other, it will be certain that the wilting of the affected plant may happen. It wil be worth to note that no suberization occurs around the diseased tissues in the affected grown plants, although this is the case at the beginning of the infection, and that the mycelia lodging in the tissues of the grown plants may intrude rather easily into the host cells piercing directly the cell walls at their weaker points, though in the case of the beginning of the disease, the mycelia grow up rather intercellulary (Fig. 4, 5, 6). These differences are considered to be mainly due to the poisoning effect upon the host tissues of a certain substance or substances1) excreted from the mycelia growing vigorously in the tissues.1) 根端又は其附近より侵人した Fusarium niveum は間もなく心柱内全組織を崩壞し, 菌絲を以て充滿するに至る。多くの幼根が斯の如く激しく侵さるる時は細根による根壓の現象は著しく減退するが故に, もしこの際土壤水分が一定度以下に減退する時は麥凋現象の發現となるのである。2) 被害甚しからざる根より新側根が形成せらるる時は, 新根の多くは根端が未だ抽出しない内に母根内に於て感染するのである。3) 被害を受けた莖又は地際の根に於ては, 最初その木質部に被害を見るのであるが, 組織はその形態に變化を來すことなく單に組織構成柔細胞の内容の崩壞を見るのみである。被害甚しきに至れば組織は乾燥し褐色を呈するに至る。被害組織内の導管内に存する異物としては, 菌絲, 樹脂樣物質及び填充體を擧げ得るが, これらは何れも導管の水分通導を阻碍する原因とはならないものであることを認めた。4) 被害皮部柔組織及び靱皮部は次第に崩壞して褐色を呈し扁平となる。而して射出髓,髓及び形成層部は崩壞してついに空洞と化し, その内腔に樹脂樣物質の集積を見ることが稀でない。5) 根の先端部より侵人したる病原菌は, ついに寄主の導管内に入り, かくて後 micoconidia の轉流によつて遠方に轉移し, かくて後導管の周壁を破つて周在組織を侵すのである。かくの如くしてついに導管周在組織は全くその形骸のみを殘すに至る結果, これによつて保護せられたる導管の水分通導機能は遂行不可能の状態に隔る筈である。これらの病變が單に一個又は少數の導管に於て發生するに止まらず, 一定數以上の維管束に於て且つ相互に略相連絡して起る時は即ち麥凋現象の發現となるのである。
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