INTRODUCTION 5 



unhealthy ; it is only when the life-processes have sunk to very 

 small proportions that we speak of a plant as " sickly." 



Such sickly plants recover, as a rule, when the deficiency of 

 light, heat, nutriment, or whatever the cause of the sickliness 

 may be, is removed. It is the province of physiology to discover 

 the conditions under which plants thrive best. I do not regard 

 the investigation of the phenomena of mere sickliness as the task 

 of pathology. It is only when the sickly condition leads to the 

 death of some part of the plant that we may speak of actual 

 disease. Suppose, for instance, that the soil of a wood has 

 suffered through removal of litter, a diminution of growth will 

 result, which, however, is not as yet disease ; but if a moribund 

 condition of the tops of the trees sets in, we are confronted with 

 the disease known as "top-drying" or "top-drought." This 

 example shows how gradually the condition of sickliness merges 

 into that of disease, and how it is only the partial death of the 

 plant that can be regarded as giving external indication of the 

 latter. 



It is quite as difficult to draw the boundary line between healthy 

 and diseased, and between normal and abnormal, in the case of 

 those phenomena which we are accustomed to designate as 

 monstrosities. In the nature of organisms there is a tendency 

 towards variation both morphologically and physiologically, and 

 it is upon this, in fact, that progressive evolution in the organic 

 world depends. 



Variation is, therefore, a normal phenomenon, and depends on 

 causes which are probably almost always operative in the earliest 

 stages of the life of the organism before, during, and immediately 

 after the fertilization of the oosphere. 



It is impossible to establish a strict line of demarcation between 

 normal variation and malformation ; and thus all the phenomena 

 connected with the latter, which we are not in a position to 

 explain, have been separated and grouped together to form the 

 special study of teratology apart from pathology. 



In this text-book therefore we shall confine ourselves essen- 

 tially to describing and explaining those phenomena which bring 

 about the premature death of the plant, or of any part of it, how- 

 ever small. 



This limitation leads us to the answer to the question whether 



