;68 



NATURE 



{August 1 6, 1888 



ments is termed a hypha, and it consists of a sort of fine 

 cylindrical pipe with very thin membranous walls, and 

 filled with watery protoplasm. These hyphae possess the 

 power of boring their way in and between the cell-walls 

 of the young beech seedling, and of absorbing from the 

 latter certain of the contents of the cells. This is accom- 

 plished by the hyphae putting forth a number of minute 

 organs like suckers into the cells of the seedling, and these 

 suckers take up substances from the latter : this exhaustion 

 process leads to the death of the cells, and it is easy to see 

 how the destruction of the seedling results when thousands 

 of these hyphae are at work. 



At the outer parts of the diseased spots on the 

 cotyledons or leaves of the seedling, the above-named 

 hyphae are seen to pass to the epidermis, and make 

 their way to the exterior : this they do either by passing 

 out through the openings of the stomata, or by simply 

 boring through the cell-walls (Fig. 37). This process of 

 boring through the cell- walls is due to the action of a solvent 

 substance excreted by the growing tip of the hypha : the 



Fig. 37. — Portion of a co:yledon of the beech, infested with Phytophthora 

 omiiivora : the piece is s^own partly in vertical section. The myce- 

 lium, spreading between the cells, puts forth aerial hyphae, which bore 

 between the cells of the epidermis, b and d, or emerge from the stomata, 

 a, and form coniiia at their apices : the various stages of development 

 are shown. On other hyphae, between the cells of the interior, the 

 oospores are formed in oogonia, c and f. (Highly magnified.) 



protoplasm secretes a ferment, which passes out, and 

 enables the tip to corrode or dissolve away the substance 

 of the cell-walls. It is also characteristic of these hyphae 

 that they make their way in the substance of the cell-walls, 

 in what is known as the " middle lamella * : in this, and 

 in what follows, they present many points of resemblance 

 to the potato-disease fungus, which is closely allied to 

 Phytophthora omnivora. 



The hyphae which project from the epidermis into the 

 damp air proceed to develop certain spores, known as 

 the conidia, which are capable of at once germinating 

 and spreading the disease. These conidia are essentially 

 nothing but the swollen ends of branches of these free 

 hyphae : the ends swell up and large quantities of proto- 

 plasm pass into them, and when they have attained a 

 certain size, the pear-shaped bodies fall off, or are blown 

 or knocked off. 



Now the points to be emphasized here are, not so 

 much the details of the spore-formation, as the facts that 



' (1) many thousands of these spores may be formed in 

 the course of a day or two in warm, damp weather ; and 

 (2) any spore which is carried by wind, rain, or a passing 

 object to a healthy seedling may infect it (in the way to 

 be described) within a few hours, because the spore is 

 capable of beginning to germinate at once in a drop of 

 rain or dew. A little reflection will show that this explains 

 how it is that the disease is spread in patches from centres, 

 and also why the spread is so rapid in close, damp 

 weather. 



When a conidium germinates in a drop of dew for 

 instance, the normal process is as follows. The proto- 

 plasm in the interior of the pear-shaped conidium becomes 

 divided up into about twenty or thirty little rounded 

 naked masses, each of which is capable of very rapid 

 swimming movements ; then the apex of the conidium 

 bursts, and lets these minute motile zoospores, as they are 

 called, escape (Fig. 38, a). 



Each zoospore then swims about for from half an hour 

 to several hours in the film of water on the surface of the 

 epidermis, and at length conies to rest somewhere. Let 

 us suppose this to be on a cotyledon, or on the stem or 

 root. In a short time, perhaps half an hour, the little 



Fig. 38. — Porcio'n of epidermis of a beach-seedling, on which the conidia of 

 the Phytophthora have fallen and burst, a and d, emitting the motile 

 zoospores, b, which s »on come to rest and germinate, c, by putting 

 forth a minute germinal hypha, c. e, which penetrates between the cells 

 of the epidermis, e and f, and forms the mycelium in the tissues be- 

 neath. At d a ziospore has germinated, without escaping from the 

 conidium. (Highly magn.fied : partly after De Bary and Hartig.) 



zoospore begins to grow out at one point — or even at more 

 than one — and the protuberance which grows out simply 

 bores its way directly through the cell-wall of the seedling, 

 and forms a cylindrical hypha inside (Fig. 38, b, c, e,f) : 

 this hypha then branches, and soon proceeds to destroy 

 the cells and tissues of this seedling. The whole process 

 of germination, and the entrance of the fungus into the 

 tissues, up to the time when it in its turn puts out spore- 

 bearing hyphae again, only occupies about four days during 

 the moist warm weather in May, June, and early in July. 

 We are now in a position to make a few remarks which 

 will enable practical people to draw helpful conclusions 

 from what has been stated. Let us suppose a seed-bed 

 several feet long and about three feet wide, and containing 

 some thousands of young beech seedlings : then suppose 

 — by any means whatever — that a single conidium of 

 Phytophthora omnivora is carried on to a cotyledon of 

 one of the seedlings. Let us further assume that this 

 occurs one warm evening in May or June. During the 

 night, as the air cools, the cotyledon will be covered with 

 a film or drops of water, and the conidium will germinate, 

 and allow, say, thirty zoospores to escape. Now, the 



