76 DISEASES OF TROPICAL PLANTS CH . 



irritation causes an increased and irregular growth of 

 the root which results in the formation of the gall. 

 The worms make some increase in length, and a very 

 great increase in diameter, becoming spindle-shaped 

 and finally club-shaped, the head being the small end. 

 The males undergo another transformation, becoming 

 slender, which enables them to travel through the tissues 

 of the gall until they reach the females, with which 

 they copulate, and then perish. After copulation the 

 females develop eggs from which the next generation 

 of worms is hatched. Stone and Smith give the 

 length of life of males as four weeks and of females as 

 six weeks. 



ENVIRONMENT AND FUNCTIONAL OR PHYSIOLOGICAL 

 DISEASES 



Many diseases are not due to any of the specific 

 organisms which have been discussed, but to the fact 

 that the plants are grown under unfavourable conditions. 

 Plants which are grown under such conditions lack 

 vigour and vitality and cannot make maximum growths 

 or produce the desired abundance of grain, fruit, or 

 vegetable, they are also far more susceptible to the 

 attacks of fungi, bacteria, etc., than plants that are in 

 a normally healthy condition. The factors which may 

 be unfavourable to plant growth are, soil, moisture, 

 temperature, gas, and smoke. 



The soil may be suitable to certain classes of plants 

 and unfavourable to others. Of course, it is conceded 

 that the soil must contain the necessary food elements 

 which are essential for plant growth, but frequently 

 the soil fulfils this condition and yet for some un- 

 explainable reason the plants will not thrive as they 

 should. New soils may be well suited to certain crops 

 and very unsatisfactory for others, which if grown in 

 these conditions are not only poor but also subject to 

 other diseases. 



Soils which have been kept continuously in one 



