72 



PROTECTION OF PLANTS, 1921-22 



sight. The above symptoms are typical for Green ^Mountain potatoes. It 

 might be added for those readers who have at their disposal a microscope that 

 a thin freehand section through a light green area bordered with the dark green 

 will show, even under the low power, that in the light part the palisade tissue 

 has not been able to develop normalh'. The cells instead of being from four to 

 six times as long as wide are approximately cubical or distinctly shorter. 



The Irish Cobbler, on the other 

 hand, exhibits different characteris- 

 tics. Here mottling is not usual 

 but instead the leaves of affected 

 plants are extremel}' wrinkled and 

 dwarfed. The leaflets are smaller, 

 the petioles are reduced and even 

 the haulms are dwarfed This gives 

 rise to a type known as Curly Dvjarf 

 in extreme cases. The Rural varie- 

 ties also show this group of symp- 

 toms. 



With regard to locality it is inter- 

 esting to note that with the same 

 variety the symptoms differ accord- 

 ing to whether the variety is planted 

 in a northerly or cooler district, or 

 in a more southerly, warmer place. 

 In the cooler locality mottling is pronounced whereas in the southerly, 

 warmer district mottling is reduced or altogether obscured. That the plants 

 are still diseased, however, is proved by the fact that if tubers from a plant 

 grown under the latter conditions be planted further north mottling is again 

 pronounced. 



In addition to the above symptoms it is to be noticed that plants severely 

 affected with mosaic will show this in weaker stem development and in a gener- 

 allv unthriftv condition. 



Fig. 2— Plant A is healthy. Plant B is 

 suffering with mosaic. Note at 1 the 

 scars left by fallen lea^ es and at 2 a leaf 

 which has just dropped. 



Eflfect on the Plant 



One effect of the disease has already been mentioned, namely the modified 

 growth of the palisade cells in light green areas. Other cells are affected to a less 

 noticeable extent but the general effect is that the tissues which should be manu- 

 facturing a maximdm amount of starch are not able to do so, and further a pro- 

 portion of that manufactured is undoubtedly used by the causal principle. This is 

 postulating tliat the causal principle is a parasite. Hence the amount of food 



