402 POPULAR SCIENCE MONTHLY. 



rudimentary principles and discoveries of the preceding periods. It 

 was during this period that the most spectacular conquests were made ; 

 that popularization and extension of methods occurred. So great, so 

 numerous, so wonderful were the advances made during the past 

 decade, that we frequently see the statement that little or no progress 

 had been made in plant pathology prior to 1885. The present day 

 student should, however, bear in mind that it was the persistent, 

 arduous, patient work of the preceding years that rendered possible 

 the progress of the closing years of the century. 



My denomination of this period as ' the period of growth ' indicates 

 the nature of the changes which it inaugurates; growth in every direc- 

 tion and concerning every phase of the subject. There has been growth 

 in the list of plant maladies. New diseases have been discovered 

 by scores, and old diseases have been found to affect new plants, 

 and diseases hitherto insignificant have taken prominent places 

 as dangerous foes. The alteration of the plant constitution by 

 high selection and breeding, the bringing of plants into new climatic 

 or soil relations, the more intensive cultivation, the bringing of a 

 susceptible plant into a region where a parasite is already growing 

 upon one of its botanical relatives, thus exposing it to a possible new 

 foe, are conditions that operate to admit of the evolution of new dis- 

 eases. The growing of plants in large quantities in solid blocks, 

 rather than sparingly in scattered gardens, brings about a congested 

 condition comparable with the crowding of our cities, and favors the 

 development of epidemics* by furnishing abundant material for the 

 parasitic organisms to attack, abundant nutriment upon which they 

 may multiply, and abundant opportunity for them to reach new hosts 

 and spread the contagion. With potatoes, for example, raised merely 

 as garden crops, the probability of an epidemic affecting the majority 

 of gardens is not so great as when potatoes are raised in vast fields. A 

 single field crop, once infested, so contaminates the air with spores 

 that other fields are almost sure to become infected. The contagium 

 becomes sufficiently multiplied to break the quarantine, and a general 

 epidemic results. Any factor which tends to increase the occurrence 

 of epidemics may quickly raise a given disease from obscurity to a 

 position of commanding importance. So too does the increase in 

 value of hitherto comparatively insignificant crops. The pecan and 

 cranberry are at present objects of particular solicitude by the plant 

 physician. 



With the importation of plants from foreign countries and the 

 transportation of plants from one part of the country to another comes 



* The use of the word epidemic in relation to plant diseases while etymo- 

 logically incorrect, seems justified since no other word conveys the desired 

 meaning and the meaning of this word is clear to all. 



