Augtist 1 6, 1888] 



NATURE 



3 5 9 



average size of a conidium is about 1/400 of an inch long 

 by about 1/700 of an inch broad, and we may take 

 the zoospore as about 1/2000 of an inch in diameter ; thus 

 it is easy to see that the film of moisture on the cotyledon 

 is to a zoospore like a large pond or lake to a minnow, 

 and the tiny zoospores, after flitting about in all direc- 

 tions, come to rest at so many distant points on the 

 cotyledon — or some of them may have travelled abroad 

 along the moist stem, or along a contiguous leaf, &c. 

 Before daylight, each of these thirty zoospores may have 

 put forth a filament which bores between the cells of the 

 cotyledon, and begins to grow and branch in the tissues, 

 destroying those cell-contents which it does not directly 

 absorb, and so producing the discoloured disease-patches 

 referred to. Supposing the weather to remain damp and 

 warm, some of the hyphae may begin to emerge again 

 from the diseased and dying seedling on the fourth day 

 after infection — or at any rate within the week — and this 

 may go on hour after hour and day after day for several 

 weeks, each hypha producing two or more conidia within a 

 few hours of its emergence ; hence hundreds of thousands 

 of conidia may be formed in the course of a few days, and if 

 we reflect how light the conidia are, and how their zoospores 

 can flit about to considerable distances, it is not surprising 

 that many of them are shed on to the surrounding seed- 

 lings, to repeat the story. If we further bear in mind that 

 not only every puff of wind, but every drop of rain, every 

 beetle, or fly, or mouse, &c, which shakes the diseased 

 seedling may either shake conidia on to the next nearest 



Fig. 39. — An oogonium and antheridium of Phytophthora omnivora. The 

 oogonium is the larger rounded body, borne on a branch of the myce- 

 lium : it contains an oosphere, in process of being fertilized by the proto- 

 plasm of the antheridium (the smaller body applied to the side of the 

 oogonium). The antheridium has pierced the wall of the oogonium, by 

 means of a fertilizing tube, through which the contents pass into the 

 oosphere, converting the latter into an oospore. (Very highly magnified : 

 after De Bary.) 



seedlings or even carry them further, it is clearly intelli- 

 gible how the infection is brought about, and spreads 

 through the seed-bed, gathering strength, as it were, hour 

 by hour. 



But, although we have explained the rapid infection from 

 plant to plant, it still remains to see how it is that if we 

 sow the seeds in this bed next year, the seedlings are 

 almost certain to be generally and badly attacked with the 

 disease at a very early stage. 



When the fungus-mycelium in the cotyledons and other 

 parts of the diseased seedlings has become fully developed, 

 and has given off thousands of the conidia' above described, 

 many of the branches in the dying tissues commence to 

 form another kind of spore altogether, and known as an 

 oospore, or egg-like spore. This spore differs from the 

 conidium in size, shape, and position, as well as in its 

 mode of development and further behaviour, and if it 

 were not that several observers have seen its formation on 

 the same hyphae as those which give rise to the conidia, 

 it might be doubted by a beginner whether it really 

 belongs to our fungus at all. As it is absolutely certain, 

 however, that the oospore on germination gives rise to the 

 fungus we are considering, the reader may rest satisfied 

 on that point. 



The spore in question is formed in a swelling of the 

 free end of a branch of the hypha as follows. The proto- 



plasm in the rounded end of the hypha becomes collected 

 into a ball (the egg cell or oosphere) and then a smaller 

 branch with a distinct origin applies itself to the outside 

 of this rounded swelling and pierces its wall by means 

 of a narrow tube : protoplasm from the smaller branch 

 {antheridium) is then poured through the tube into the 

 " egg-cell," which thus becomes a fertilized " egg-spore " 

 or oospore. This oospore then acquires a very hard coat- 

 ing, and possesses the remarkable peculiarity that it may 

 be kept in a dormant state for months and even a year or 

 more before it need germinate : for this reason it is often 

 called a resting spore. It has been found that about 

 700,000 oospores may be formed in one cotyledon, and a 

 handful of the infected soil sufficed to kill 8000 seedlings. 



Now, when we know this, and reflect that thousands of 

 these oospores are formed in the rotting seedlings and are 

 washed into the soil of the seed-bed by the rain, it is 

 intelligible why this seed-bed is infected. If seeds are 

 sown there the next spring, the young seedlings are 

 attacked as soon as they come up. These oospores are, 

 in fact, produced in order that the fungus shall not die out 

 as soon as it has exhausted the current year's supply of 

 seedlings ; whereas the conidia, which soon lose their 

 power of germinating, are the means by which the para- 

 site rapidly extends itself when the conditions are most 

 favourable for its development and well-being. 



It has already been mentioned that other plants besides 

 the beech are destroyed by the ravages of this fungus. 

 Not only has it been found to grow on herbaceous plants, 

 such as Sempervivum, Clarkia, and many others, but it 

 habitually attacks the seedlings of many timber-trees, such 

 as, for instance, those of the spruce and silver firs, the 

 Scotch pine, the Austrian and Weymouth pines, the larch, 

 the maples, and particularly those of the beech. 



It is obvious that this makes the question of combating 

 this disease a difficult one, and the matter is by no means 

 simplified when we learn that the fungus can live for a 

 long time in the soil as a saprophyte, and apart from the 

 seedlings. In view of all the facts, let us see, however, if 

 anything can be devised of the nature of precautionary 

 measures. It must at least be conceded that we gain a 

 good deal by knowing so much as we do of the habits of 

 this foe. 



In the first place, it will occur to everybody never to 

 use the same seed-bed twice ; but it may be added that 

 this precaution need not be taken as applying to anything 

 but seeds and seedlings. Young plants, after the first or 

 second year, are not attacked by the fungus — or rather 

 are attacked in vain, if at all — and so the old beds 

 may be employed for planting purposes. In the event 

 of a patch of diseased seedlings being found in the seed- 

 bed, as in our illustration quoted above, the procedure is 

 as follows : cover the whole patch with soil as quietly and 

 quickly as possible, for obviously this will be safer than 

 lifting and shaking the spore-laden plantlets. If, however, 

 the sharp eye of an intelligent gardener or forester detects 

 one or two isolated seedlings showing the early stages of 

 the disease, it is possible to remove the single specimens 

 and burn them, care being taken that the fingers, &c, do 

 not rub off spores on to other seedlings. 



In the last event, the beds must be looked to every day 

 to see that the disease is not spreading. All undue 

 shading must be removed, and light and air allowed free 

 play during part of the day at least ; by such precautions, 

 carefully practised in view of the above facts and their 

 consequences, it is quite feasible to eradicate the disease 

 in cases where ignorant or stupid mismanagement would 

 result in the loss of valuable plants and time. In the 

 case of other seedlings also, much may be done by 

 intelligently applying our knowledge of the disease and 

 its cause. It is not our purpose at present to deal with 

 the diseases of garden-plants, &c, but it may be remarked 

 in passing that in the large majority of cases the " damp- 

 ing off " of seedlings is due to the triumphant development 



