Text-book of structural and physiological botany . dry gangrene, according as the product of decomposi-tion of the cellulose is a fluid or a solid substance. Thelatter form very commonly first attacks the innermost layersof wood, which may be, according to the colour of the sub-stance formed, red or white. Plants attacked by decay may The Life of the Plant, 225 still live a long time under certain circumstances, and mayhave a perfectly sound appearance on the outside, untilthe disease reaches the cambium-layer, when they quicklyperish. The decomposition of cellulose does not, however,always be
Text-book of structural and physiological botany . dry gangrene, according as the product of decomposi-tion of the cellulose is a fluid or a solid substance. Thelatter form very commonly first attacks the innermost layersof wood, which may be, according to the colour of the sub-stance formed, red or white. Plants attacked by decay may The Life of the Plant, 225 still live a long time under certain circumstances, and mayhave a perfectly sound appearance on the outside, untilthe disease reaches the cambium-layer, when they quicklyperish. The decomposition of cellulose does not, however,always begin inside the plant; it may also penetrate fromwithout, at spots which have become diseased from ex-ternal injury. Diseases of another class, produced by vegetable/<2r^i/to,that is by plants which derive their nourishment from theliving tissue of the plants they inhabit, are in general lessobscure than those which were last described. Among these parasites are certain Fungi (Fig. 374),which are of the greatest practical importance. The ques-. FiG. 374.—Destruction of the cell-wall caused by a Fungus : transverse section of thewood of a beech \ zX a, a the destruction of the secondary layers is still in pro-gress , h,b the entire cell-wall has been destroyed with the exception of alarger or smaller residue of the primary layer, and the cell-cavity filled withan amorphous substance. tion which has often and for a long time been discussed re-specting these plants, whether they are the cause or only theaccompaniment of the disease, has now been decided infavour of the former alternative, since it has been provedthat the organisms in question are well-defined species ofFungus, and that the same organism is always found to ac-company the same disease. In addition to this, it has been Q 22 5 Structural and Physio log ical Botany, proved by a number of observations and careful experi-ments, that these diseases are contagious, and that healthyplants become subject to a particu
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