182 Scientific Proceedings^ Royal Dublin Society. 



bush, eight feet high, which was heavily infested with aphides, grew within ten 

 feet of the potatoes; and from this the insects dispersed to some extent to 

 neighbouring comparativelj' non-infested rose trees, but only to a very slight 

 extent to the potatoes. 



An effective fumigant for aphid destruction. — It has been stated that the 

 development of aphides was feared during winter storage on the sprouts of the 

 various seed potatoes intended for the experimental disease plots. This un- 

 fortunately began to take place early in March, 1922 ; and after recourse had 

 been had to a twice-repeated fumigation with commercial fumigants without 

 completely satisfactory results, a successful method of treatment was discovered. 

 The sprouting boxes containing the seed potatoes were placed in a small room 

 having a capacity of 1,700 cubic feet. One pound of tetrachlorethane was then 

 distributed in small lots in earthenware saucers in various parts of the room and 

 allowed to evaporate. The room was then closed up, and was not opened for 

 three days. The effect was immediate and lasting, for no more living aphides 

 were seen up to the time planting was concluded, about thirty-three days later. 

 The vapour of this chemical does not injure the tubers or sprouts, and it is 

 perfectly safe and not unpleasant to use. 



VII. — Summary. 



It was established that the presence of an excess of starch in the rolled 

 leaves of diseased plants is a constant symptom of leaf-roll. 



The rolling of the leaves of diseased plants was found to be preceded by 

 the accumulation of starch in the mesophyll. 



The artificial darkening of diseased plants before their leaves rolled, and the 

 consequent reduction of photosynthesis to a minimum, was found to prevent the 

 rolling of the leaves for long periods. 



Temporary rolling of the leaves of healthy plants was brought about by 

 depriving the latter of most of their growing points and storage organs. Accom- 

 panying the rolling a great excess of starch was found in the rolled leaves. The 

 rolling and excess of starch afterwards disappeared when normal gro^i:h was 

 allowed to proceed. 



It is concluded that rolling of the leaves is a direct consequence of the 

 presence in them of an abnormal amount of starch, and probably of other carbo- 

 hydrate, and that it is caused by the distention of the spongy parenchyma, which 

 was demonstrated. 



Starch accumulation in the leaves accompanies rolling due to some other 

 causes, such as injury to the base of the stalk, attacks of black-stalk rot, and 

 other obscure disturbances. 



Evidence is presented to show that the seat of the disturbance in the trans- 

 location of carbohydrate from the leaves of diseased plants resides in the blades 

 of the leaves, where the accumulation of starch begins and ends, and not in the 

 disorganization of the phloem in distant tissues. 



Low temperatures were found incapable of causing healthy leaves of a living 

 plant to accumulate starch or to roll. 



The presence of disorganized phloem was established in plants attacked by 

 PhytoplitJiora infestans and in others apparently suffering from an attack of 

 eel-worms. 



The disappearance of the starch in diseased leaflets proceeds from base to tip, 

 but in healthy leaflets from tip to base. 



The brown spots which develop on affected leaves originate in the death of a 

 single cell of the epidermis. 



