92 Minnesota Plant Diseases, 



however, have a limited life. A plant evinces two processes 

 constantly at work, viz. : a constructive process which is build- 

 ing up the tissues, increasing or replacing them, and a repel- 

 lant process which organizes and otherwise provides for the 

 repulse of unfavorable conditions among which may be includ- 

 ed the attacks of injurious weather and soil conditions as well 

 as those of fungi and other organisms. Now in the normal 

 vigor of youth a plant is capable not only of successfully re- 

 pelling external attacks but puts much energy into the con- 

 structive work. Gradually less and less of an increase of tis- 

 sues is noticeable because of the necessary replacement of lost 

 tissues and finally we reach the mature vegetative condition of 

 a plant where the latter has attained its greatest possible size 

 and all of its constructive power is exerted to replace lost mem- 

 bers or parts. If conditions were ideal, one might imagine such 

 a mature or prime condition to last indefinitely, but now with 

 the increase in size and complexity we find also an increase in 

 the attacks of foreign organisms or the unfavorable conditions 

 due to accidents, as lightning strokes or storm damage. If the 

 repelling power of the plant does not increase, the disintegrat- 

 ing forces gain and the plant enters the period of old age and 

 decline which is terminated only by the complete success of 

 the disintegrating forces, i. e., the death of the plant. Among 

 the shortest-lived individuals are those plants which live but a 

 single season. Among the longest-lived are not only our giant 

 trees but also those herbaceous plants which have creeping 

 underground stems, that travel from year to year, carrying 

 their reserve material as a capital for starting work again in the 

 following spring. Such plants as iris, bloodroot and many of our 

 grasses are good examples of such long-lived plants. The at- 

 tack of some foreign organism has in general more chance of 

 success during the old age period than during the vigor of 

 youth and old age in a plant, therefore in general predisposes 

 that plant towards disease. 



Factors of disease. Disease in plants has these three fac- 

 tors : first, the immediate cause of disturbance, as a fungus or 

 some insect or some unfavorable atmospheric agency; sec- 

 ond, the resultant change in the life of the host ; and third, the 

 previous condition of the host plant, i. e., its predisposition in 



