176 Scientific Proceedings, Royal Duhlin Society. 



the first instance at or near the tip, and then gradually vanishes from the lower 

 portions. Here there is not the same sharp line bounding the starch-free area, 

 and the progress of hydrolysis is not so regular, but the direction in which it 

 proceeds is unmistakable. Furthermore, in the case of healthy leaves the starch 

 disappears first from the tip of the terminal leaflet, then from the tips of the next 

 pair, and so on to the base of the leaf; each leaflet at any one time showing a 

 somewhat larger area still full of starch than the leaflet immediately above. 

 This method of evacuation is also followed by healthy leaves and leaflets which 

 contain an excess of starch, due to causes already discussed, and which roll 

 upwards in consequence (figs. 6 and 7, PI. VI). 



The brown areas which generally arise in the course of time on rolled leaves 

 appear to originate in the death of a single cell either of the upper or the lower 

 epidermis, this being most frequently a g-uard-cell, but not invariably so. The 

 walls of the cell become brown as well as the contents, and the discoloration 

 spreads to the wall of any neighbouring cell. The cells involved collapse and 

 fall in, particularly when the affected area starts on the lower surface of the leaf. 

 All the subjacent tissues of the mesophyll are liable to be attacked in turn, 

 until the lesion extends from one side of the leaf to the other. No parasite was 

 seen in connexion with this injur}'. It 7nay be remarked that tlie cells of the 

 epidermis of diseased leaves contain a]iparently more vigorous nuclei, more 

 abundant cytoplasm, and a greater quantity of starch than corresponding cells 

 of healthj' leaves. The difference is more marked in the case of the upper 

 epidermis. 



VI. — Insect Carriers of Leaf-Eoll. 



A survey, which was unavoidably somewhat hurried, was made by my 

 colleague, Mr. Ehynehart, during July, 1921, of the common insects which 

 occurred in the experimental potato disease plots; and the writer wishes to 

 acknowledge the help thus given. The following insects were found on the 

 plants, and were presumed to be feeding on them : — 



Galocoris bipunctatus (Capsid Bug). Abundant. 

 Typlilocyha Ulmi (Jassid). Abundant. 

 Philaneus sptimarius (Frog-hopper). Abundant. 

 Psylliodes affiyiis (Potato flea-beetle). AlDundant. 

 Aphides (Genus and sp. not determined). Fairly common. 

 Lygus pratensis var. canipesiris (Capsid), Fairly common. 

 Typlilocyha sp. (Jassid). Fairly common. 

 Anthocorus sylvestris (Anthoeorid). Scarce. 

 Aetoi'Mmis angulatus (Capsid). Scarce. 

 Idiocerus sp. (Jassid). Rare. 

 Bytlioscopus sp. (Jassid). Rare. 



Among these insects aphides certainly appeared to be of secondary impor- 

 tance. They were unevenly distributed in the plots, being almost absent in 

 some parts.' This may have been due to the great prevalence of ladybirds 

 (Coccinella sp.) and their larvae in the month of June, these being practically 

 the only insects then to be found on the foliage. The same prevalence of these 

 beetles "and their larvae and scarcity of aphides was noted in 1922. Estimating 

 the aphides individual for individual against the other kinds of insects, they 

 should probably be classed as "fairly common" on the average in 1921. 



Infection experiments with insects other than aphides. — Experiments to test 



