August 16, 1888] 



NA TURE 



3^7 



of object. These strange and, at present, unaccountable 

 resemblances will probably be developed and possibly be 

 interpreted by future investigations. 



A. M. Clerke. 



TIMBER, AND SOME OF ITS DISEASES. 1 



XI. 



T T may possibly be objected that the subject of the pre- 

 -*■ sent paper cannot properly be brought under the title 

 of these articles, since the disease to be discussed is not a 

 disease of timber in esse but only of timber in posse ; 

 nevertheless, while acknowledging the validity of the 

 objection, I submit that in view of the fact that the malady 

 to be described effects such important damage to the 

 young plants of several of our timber-trees, and that it is 

 a type of a somewhat large class of diseases, the slight 

 impropriety in the wording of the general title may be 

 overlooked. 



It has long been known to forest nursery-men that, when 

 the seedling beeches first appear above the ground, large 

 numbers of them die off in a peculiar manner — they are 

 frequently said to " damp off" or to "rot off." A large class 

 of diseases of this kind is only toD familiar, in its effects, 

 to cultivators in all parts of the world. Every gardener, 

 probably, knows how crowded seedlings suffer, especially 

 if kept a trifle too damp or too shaded, and I have a dis- 

 tinct recollection of the havoc caused by the "damping 

 off" of young and valuable Cinchona seedlings in Ceylon. 

 In the vast majority of the cases examined, the" damp- 

 ing off" of seedlings is due to the ravages of fungi 

 belonging to several genera of the same family as the one 

 {Phytophthora infestans) which causes the dreaded potato 

 disease— i.e. to the family of the Peronosporeae — and since 

 the particular species {Phytophthora omnivord) which 

 causes the wholesale destruction of the seedlings of the 

 beech is widely distributed, and brings disaster to many 

 other plants ; and since, moreover, it has been thoroughly 

 examined by various observers, including De Bary, Hartig, 

 Cohn, and others, I propose to describe it as a type of the 

 similar forms scattered all over the world. 



It should be premised that, when speaking of this disease, 

 it is not intended to include those cases of literal damping 

 off caused by stagnant water in ill-drained seed-beds, or 

 those cases where insufficient light causes the. long-drawn, 

 pale seedlings to perish from want of those nutrient sub- 

 stances which it can only obtain, after a certain stage of 

 germination, by means of the normal activity of its own 

 green cotyledons or leaves, properly exposed to light, air, 

 &c. At the same time, it is not to be forgotten that, as 

 conditions which favour the spread of the disease to be 

 described, the above factors and others of equal moment 

 have to be taken into account ; which is indeed merely 

 part of a more general statement, viz. that, to understand 

 the cause and progress of a disease, we must learn all we 

 can about the conditions to which the organisms are ex- 

 posed, as well as the structure, &c, of the organisms 

 themselves. 



First, a few words as to the general symptoms of the 

 disease in question. In the seed-beds, it is often first 

 noticeable in that patches of seedlings here and there 

 begin to fall over, as if they had been bitten or cut where 

 the young stem and root join, at the surface of the ground : 

 on pulling up one of the injured seedlings, the " collar," or 

 region common to stem and root, will be found to be 

 blackened, and either rotten or shrivelled, according to the 

 dampness or dryness of the surface of the soil. Some- 

 times the whole of the young root will be rotting off before 

 the first true leaves have emerged from between the coty- 

 ledons ; in other cases, the "collar" only is rotten, or 

 shrivelled, and the weight of the parts above ground 



1 Continued from p. 29^. 



causes them to fall prostrate on the surface of the soil ; in 

 yet others, the lower parts of the stem of the older seed- 

 ling may be blackened, and dark flecks appear on the 

 cotyledons and young leaves, which may also be turning 

 brown and shrivelling up (Fig. 36). 



If the weather is moist — e.g. during a rainy May or 

 June — the disease may be observed spreading rapidly from 

 a given centre or centres, in ever-widening circles. It has 

 also been noticed that if a moving body passes across a 

 diseased patch into the neighbouring healthy seedlings, 

 the disease in a few hours is observed spreading in its 

 track. It has also been found that if seeds are again 

 sown in the following season in a seed-bed which had pre- 

 viously contained many of the above diseased seedlings, 

 the new seedlings will inevitably be killed by this "damp- 

 ing off." As we shall see shortly, this is because the rest- 

 ing spores of the fungus remain dormant in the soil after 

 the death of the seedlings. 



Fig. 35. — A young beech-seedling attacked by Phytophthora. omnivora: the 

 moribund tissues in the brown and black patches on the young stem, 

 cotyledons, and leaves, are a prey to the fungus, the mycelium of which 

 is spreading from the different centres. The horizontal line denotes the 

 surface of the soil. 



In other words, the disease is infectious, and spreads 

 centrifugally from one diseased seedling to another, or 

 from one crop to another : if the weather is moist and 

 warm — "muggy," as it is often termed — such as often 

 occurs in the cloudy days of a wet May or June, the spread 

 of the disease may be so rapid that every plant in the 

 bed is infected in the course of two or three days, and the 

 whole sowing reduced to a putrid mass ; in drier seasons 

 and soils, the spread of the infection may be slower, and 

 only a patch here and there die off, the diseased parts 

 shrivelling up rather than rotting. 



If a diseased beech seedling is lifted, and thin sections 

 of the injured spots placed under the microscope, it will 

 be found that numerous slender colourless fungus-filaments 

 are running between the cells of the tissues, branching 

 and twisting in all directions. Each of these fungus-fila- 



