July 23. 1875. ] 



JOURNAL OF HOBTICULTDRE AND COTTAGE GARDENEli. 



71 



. of June. In those leaves I detected the mj'celium and a few 

 fertile threads of the obnoxious fungus which always accom- 

 pauies, and probab'y causes (judging from its effects) the 

 Potato disease. The reason why competent observers believe 

 the fungus to cause the disease is simply this : If the spores 

 are allowed to fall artificially upon the leaves, or even stem or 

 tuber, they burst and grow, and wherever they burst (their 

 contents being extremely potent and corrosive) they immedi- 

 ately bore a hole through the skin and enter the plant. Of 

 course the spores eat their way into the plant in the same 

 unceremonious manner when they fall naturally upon it ; and 

 if leaves are kept under observation it is invariably seen that 

 wherever the damaging mycelium of the Potato fungus runs, 

 there, and there only, the Potato plant is discoloured, burnt 

 up, and corroded as if some poisououB acid had been iutrodnced 

 into its system. 



Soon after I had thus early detected the true Potato fungus 

 in the specimens sent from the ofBce of this .Journal, Mr. 

 Berkeley, at the meeting of the Royal Horticultural Society 

 held on .Tune liUh last, directed the special attention of the 

 gardening public to the early and extremely virulent nature of 

 the disease. At the same time he said ho had detected a 

 fungus new to him in the tissues of the Potato leaves, which 

 he referred to n dubious class of fungi always found growing 

 within the tissues of plants, and named Protomyces. In my 

 search for the latter body I met not only with the presumed 

 Protomyces — really the mature resting-spore — but also the 

 male and female organisms which gave it birth. How this 

 arose was detailed in the paper read before the Royal Horti- 

 cultural Society, and printed in last week's number of the 

 Journal of Horticulture. I am disposed to entirely account 

 for the excessive virulence of the disease this year from the 

 fact of these rcsting-spores being prematurely formed in the 

 substance of the young plants. They are embedded like cancers 

 in the tuber, haulm, and foliage ; and in whatever part they 

 are seen, there the cells of which the Potato is built up are 

 corroded and destroyed. On the application of moisture to 

 the plant, as supplied naturally by the atmosphere or arti- 

 ficially by watering, this spawn within the plant starts into 

 renewed life to further poison the tissues, produce fresh spores 

 for the present season and resting-spores for the next. 



With a little attention it is quite easy for any person of 

 ordinary ability to understand the nature of the Potato fungus 

 and how it aljfects and destroys the plant it lives upon. From 

 the smallness of the parasite it is almost invisible, but its 

 smallness is compensated for by the habit it has of so rapidly 

 increasing, that one infected plant on a given spot will in a 

 day or two cause the fungus to appear on every plant in the 

 neighbourhood. Because it is small it must not be considered 

 impotent or disbelieved in. The aphis is small, but none the 

 less destructive ; and it is quite as unreasonable to refer the 

 Potato disease to wireworms and other insects, as for some 

 dim-sighted person to deny the damaging effects of the aphis 

 because he could not see the insects, and charge rats with the 

 damage because he happened to see a rat — an animal he coxild 

 see — wallowing about amongst dead plants. 



A word may here be said about certain words and terms 

 used in the dcecription of various organs and structures of 

 plants. It is really melancholy to find writers who ought to 

 know better attempting to cast ridicule on the things they 

 cannot understand. One writer strong in wireworms and rats 

 may be found laughing at the pest which for good reasons is 

 supposed to cause the murrain, simply because he cannot 

 understand or see it, but the most short-sighted person can 

 of course see a big insect or a four-footed animal. Insects 

 are always attacking Cabbages, Turnips, and other kitchen- 

 garden plants, but no one has ever yet seen a Cabbage or 

 Turnip with the Potato disease, although it is common to see 

 both and all similar plants with the wireworm. 



A friend of mine, too, has recently acknowledged in print 

 that " ordinary capacities are staggered on the very threshold " 

 of the paper you printed last week by such words (amongst 

 others) as " cellular tissue " and " mycelial threads." Now, 

 surely, this is a libel on the gardening fraternity at large, for 

 if there are any men of "ordinary capacity" who buy and 

 read the horticultural papers and don't know that the spawn 

 of a fungus is composed of " mycelial threads," and that a 

 plant is entirely built up of " cells " packed together and 

 called cellular tissue (and tubes called "vessels"), then I 

 venture to say the time is fast approaching when such men 

 wUl have to retire from the honoured ranks. 



I have engraved the accompanying figure of the Potato 



fungus in its new aspect, so that all readers of the Journal of 

 Horticulture may, if they try, thoroughly understand its 

 nature and effects. First of all it must be remembered that 

 the Potato plant (like all other plants) is principally built up 

 of cells, called " cellular tissue." The present figure is a 

 greatly enlarged view of a small piece of Potato stem. Now, 

 I have little doubt every reader of this paper well knows that 

 a Potato stem is not like a solid piece of iron, or marble, or 

 glass, but that it is principally built up of a number of cells 

 filled with fluid, which when enlarged in size look like the 

 cells of a honeycomb. These honeycomb like cells are almost 

 invisible to the unaided sight, but when they are greatly en- 

 larged by a good microscope they are seen exactly as at A on 

 the accompanying engraving. The Potato fungus is prin- 

 cipally seen on the other side of the illustration, and everyone 

 who looks on the figure will observe that if the cells are next 

 to invisible without a glass, how much more so must be the 

 threads and spores of the fungus, which are drawn to the 

 same scale. 



The Potato fungus is principally composed of threads, called 

 " mycelial threads " or spawn, b b. These threadd are poison- 

 ous to the Potato plant, and wherever they go they scorch 

 and dry up the cells of which the Potato plant is composed. 

 The tLieads branch in various directions, iind the branches 

 bear " simple spores," or seeds, at their tips, c o c. These 

 spores when ripe fall from the threads, burst, enter the platjt 

 through the skin or breathing pores, aud make fresh corrosive 

 threads, which soon produce a new generation of spores. At 

 the ends of some of the branches it is common to see spores 

 of a different nature, n n. These are called " swarm spores" 

 because they are many times bigger than the simple spores, 

 aud contain within themselves a swarm of spores. On the 

 application of moisture the skin of this large spore bursts 

 and sets free, instead of one, a " swarm " of nearly twenty 

 spores. These fall again on the plant, burst, enter the skin, 

 and send out corrosive threads into the tissues just like the 

 last-mentioned ; but they differ from the simple spores in a 

 remarkable way, inasmuch as each individual of the swarm is 

 furnished with two lash-like taUs, e e. With these tails they 

 whirl themselves about in moisture, spin round and round, 

 and dart about in different directions with the greatest rapidity 

 like animalcules (for this reason they are called zoospores, 

 because they are partly animal-like). It will be seen from 

 this that when in moist weather the wind cannot carry the 

 infection by the simple spores, the moisture supplied to the 

 Potato by dew or showers enables these animalcula-like zoo- 

 spores to propel themselves in different directions from one 

 plant to another. 



The above contains the pith of all that was known about the 

 damaging Potato fungus till this year, although it has from 

 the first been suspected that the parasite had some means of 

 living during the winter, or how could it appear with such 

 regularity every summer ? This winter life is now understood, 

 and it may possibly prove one step towards the extirpation of 

 the pest. The two sorts of spores above described are very 

 fragile and short-lived. If they do not fall upon the Potato 

 plant or some close ally they speedily perish, but if they do 

 fall upon the Potato plant they speedily kill it, and they and 

 their mycelial threads perish with the victim. 



The Potato fungus lives through the winter in the ground, 

 and resides during this period in a state of sleep or hybernation 

 within a " rcsling-spore," as it is called, because this sort of 

 spore when mature does not burst and enter the plant, but 

 it surrounds itself with a hard coat (at the Potato plant's 

 expense), and falling to the ground with the dead haulm, 

 leaves, and tuber, quietly rests in the ground till the follow- 

 ing summer. 



This resting -spore is brought into being in a different manner 

 from the simple spores and zoospores, and this is how it is 

 done : Instead of spores, two difftrent-sized bodies arise from 

 the threads, one a globular semi-transparent organism, similar 

 in nature to the ovule of a flower, f r ; aud another much 

 smaller and similar in nature with an anther and its pollen, 

 GO. In a somewhat similar manner to the anther coming 

 in contact with the stigma of a flower, these two bodies come 

 together, and the material contained in the smaller body is 

 discharged through a minute tube into the matter contained 

 in the larger one. This action causes the larger body to secrete 

 a new coating as at h, and afterwards become a fertile resting- 

 spore. As it grows it speedily disengages itself from the thread 

 which has borne it, j j, and it falls to the ground wi*h ihe 

 dying plant, and rests in the earth Kke a Eeed. 



