234 THE OEIGIN AND PEOPAGATION OF DISEASE. 



So the fructification of tlie fuugtis naturally takes place upon the sur- 

 face of the leaves of the potato-vine. The spores fall off, are carried by 

 the rain into and through the soil, and so reach the potatoes beneath. 

 Next year, when the infected ijotato-eyes are planted, germination begins 

 again, the mycelium filaments grow upward through the stem and leaves, 

 and in July or August fructification appears on the exterior as before. 



This species affords a good example of the extreme fecundity of the 

 parasitic fungi. It has been estimated that, on the under surface of a' 

 potato-leaf, one square line is capable of producing over three thousand 

 spores. Each spore supplies at least six zoospores ; so that from one 

 square line we may have nearly twenty thousand reproductive bodies, 

 each capable of originating a new mycelium 5 and a square inch of sur- 

 face may yield nearly three million such bodies. 



The mycelium filaments can penetrate the cellular tissue of a leaf in 

 twelve hours, and, when established there, may grow and bear fruit in 

 eighteen hours longer, while the spores are perfected and ready to ger- 

 minate in twenty-four hours after they have been detached and placed in 

 water. This fully explains the rapidity with which the disease is known 

 to si)read. 



The subject of internal vegetable parasites is of the greater impor- 

 tance, because we now know that they may attack animals as well as 

 plants. The best illustration of these affections is perhaps the disease 

 which, under the name of pebrine, has been so destructive to the silk- 

 worm in France. Eight or ten years ago its effects were so serious that, 

 in 1865, the annual production of silk in that country was reduced to 

 less than one-sixth of its former average, and the loss in money-value 

 for that year alone amounted to twenty million dollars.* It was due 

 entirely to the influence of a microscopic vegetation, which destroyed 

 the silk-worm, and was readily communicated to the neighboring broods. 



It is j)laiu, therefore, that the study of parasitic diseases, for many 

 years, has been increasing in development and becoming of greater im- 

 portance in general pathology. From being confined, as at first, to a 

 few cases of local disorder, it has now come to embrace a great variety 

 of morbid affections. It has demonstrated the close connection existing 

 between animal and vegetable pathology, and it has shown that severe 

 and even fatal constitutional disorders of the animal frame may result 

 from the internal growth of microscopic parasites of a vegetable nature. 

 And these facts have been ascertained by patient microscopic inves- 

 tigation, and laborious experiment on the development of eggs and spores, 

 and the phenomena of infection and contagion. It cannot be denied 

 that the results, so far, are genuine. 



We now come to a part of the subject which may seem to be less 

 directly connected with medical doctrines ; and yet it is one which, if it 

 really have a bearing on x)athology, gives to the whole question a char- 

 acter of still greater importance. That is, the true nature of the process 

 oi fermentation. 



* Tyndall, "Fragments of Science," New York, 1872, p. 288. 



