25 



the apple and pear as compared with the work of other .scale insects is 

 also a case in point, the incnnise in the oxidizing- enzymwhich follows 

 its attack being enormous and the evident injury spreading- frequently 

 t) to S mm. from the point of puncture. 



Besides the irritants injected and the consequent changes which take 

 place in the neighboring cells, account must be taken of the elaborated 

 plant food extracted by the insects in question, as the waste of sac- 

 charin materials in the form of honey dew is often enormous, and the 

 proteid material used by the insects themselv(>s nuist l)e quite con- 

 sidera])le, especialfy when they are numerous. The stunting of the 

 plant is probably in part due to the loss of food material in this way. 



The work of aphides, thrips, and red spiders often produces in other 

 plants changes similar to those following their attacks on carnations. 

 This is especially true of rather thick-leaved plants, such as Bryo- 

 phvllum, tulip, etc. It was found that as a rule plants rich in oxi- 

 dizing enzyms react more strongly to punctures of this kind, and that 

 plants which have been weakened by long-continued forcing, and con- 

 sequently have made a poor, starved growth, are nuich richer in these 

 oxidizing enzyms than are the stronger-growing, more vigorous indi- 

 viduals of the same vari(>ty. It is also an interesting fact that the 

 aphides are especially fond of these weak plants and in€rease rapidl}^ 

 on them. 



EFFECT OF THE DISEASE ON THE PLANT AS A WHOLE. 



The effect of the disease on the plant as a whole depends upon the 

 individual vigor of the plant and upon the number of punctures. 

 A few punctures made ])y aphides, thrips, or red spiders will not 

 seriously injure a plant, but a large number will cause it to become 

 prematurely stunted and yellowish and also more susceptible to injury 

 from various causes, especially parasitic fungi, which attack the weak 

 stems and roots. Thrips, even though not numerous, may often 

 cause the leaves of healthy, vigorous plants to become much dis- 

 torted and show light blotches where they have been punctured. The 

 value of flowers punctured in this way is of course materially reduced. 



Red spiders seldom do much injury unless the foliage of the plants 

 has been kept very dry. The stunting of the plants which follows a 

 severe attack of any of these animal organisms brings about a prema- 

 ture ripening of the lower leaves, especially if the plants are rather 

 crowded, and such leaves gradually turn yellow and die. Any other 

 cause which stunts the plants growing under the conditions mentioned, 

 however, will bring about a similar maturing and death of the lower 

 leaves. 



The susceptibility of different varieties of carnations to the injury 

 seems to be as a rule proportionate to the normal vigor of the variety 

 under the conditions in which it is growing when attacked. The 



