226 



HARDWICKE'S SCIENCE-GOSSIP. 



These bodies make the study of. the potato disease 

 more complicated, and its ultimate eradication far 

 more difficult ; for they do not gerniiuate at once (as 

 do the spores and zoospores), or perish, but remain 

 quiescent "for a whole season, till certain favourable 

 external conditions cause them to burst from their 

 sleep and reproduce the parent. Resting spores 

 and dormant sclerotioid tuberiform bodies are very 

 common amongst fungi, a very remarkable instance 

 being found amongst the Agaracini. In Agaricus 

 tuberosus, we have an agaric springing from a tuberi- 

 form base, which is invariably found growing from 

 the dead remains of the previous year's fungi, gene- 

 rally the RussuUc ; but we have found the sclerotia at 

 the bottom of the tubes of some of the Polyporei, 

 the perfect agarics emerging through the tubes. 



Closely allied to the potato fungus is another 

 species found infecting chickweed {Stellaria media), 

 and named by Casparry Peronospora ahineanm. 

 In this species, and some others of the genus, male 

 organs, or antheridia, have been detected, as shown 

 at Q, fig. 158, where the mycelial filaments are shown 



Fig. 158. Peronospora tilsinearum, enlarged 40D diameters. 



bearing the oogone, with which the mature anthcri- 

 dium is shown in contact : the contents of these 

 cells are interchanged, and thus an oospore or resting 

 spore is produced. At E, is shown a section with 

 the inflated summit of the fecundating tube of the 

 antheridium (X) touching the gonosphere : this lat- 

 ter has a neat outline, produced by the membrane of 

 cellulose which has just been secreted. 



At fig. 159 (S) is shown a ripe oospore, furnished 

 with its thick reticulated epispore, the surrounding 

 protoplasm having almost disappeared ; and at T a 

 ripe oospore, whose epispore has been detached by 

 maceration in water ; a thick, colourless endospore 

 remaining, composed of two thick layers contain- 

 ing protoplasm, with two unequal vacuities. The 

 fecundating tube may be seen still fixed in the 

 endospore at U. These oospores, or resting spores, 

 of the chickweed parasite, like those of the potato, 

 possess the singular property of remaining dormant 

 during the winter, and germinating (under favour- 

 able circumstances) during the following season. 

 . We have now glanced at the fungus and its 



effect upon the foliage and stem ; but we are all of 

 necessity most interested in its fatal effects upon 

 the potato itself. In the vast majority of instances 

 the fungus makes its first wholesale attack upon 

 the leaves, sending its destructive mycelial threads 

 down the leaf-stalks into the stem, and thence, and 

 lastly, into the potato itself. If this takes place 

 when the potato-plants are young, growth is at once 



Fig. 150. Peronospora alsineraum, enlarged 400 diameters. 



arrested ; but if the plants are well established, the 

 tubers are found to be discoloured. This is 

 undoubtedly caused by the presence of the 

 fungus beneath the cuticle of the potato ; for 

 if the potatoes are taken up and kept in a 

 damp air for a day or so, the perfect parasite pre- 

 sents itself upon the surface. From the exterior 

 of the potato the fungus penetrates to the interior, 

 decomposing the tissues, and making the tuber a 

 suitable nidus for various other fungi, which are 

 not long in making their appearance. With the 

 decomposition comes the disgusting odour so well 

 known in connection with diseased potatoes ; the 

 diseased tuber is now attacked by insects, and its 

 end is one horrible fcetid mass. It generally 

 happens that the eyes are the last to succumb to 

 the disease ; and it is stated, that if these are cut 

 out and planted, i.hey grow into healthy plants ; 

 but if the fact is taken into consideration of the 

 resting spores being produced within the inter- 

 cellular passages of spent potatoes, and that these 

 resting spores are capable of lying dormant during 

 a whole season, it seems reasonable to imagine 

 that the planting of such eyes would be the one 

 certain means of spreading the disease. 



That the fungus attacks apparently healthy 

 plants there can be no manner of doubt, the pre- 

 vailing opinion now being that it is by no means 

 necessartj that a plant should be in ill health for a 

 fungus to find thereon a suitable nidus. Contrary 

 opinions have, however, long been held, and are 

 still held on this point, many observers thinking 

 that excessive moisture, over-cultivation (if such 

 a thing be possible), electrical influences, or 

 attacks of insects, first affect the health of the 

 plant, and predispose it to succumb before the 

 attacks of the fungus. Mr. Alfred Smee, surgeon 

 to the Bank of England, has long held his ground 

 upon the hypothesis that the potato is first attacked 



