192 DISEASE IN PLANTS. 



copper-coloured as they dry ; the damage is 

 due to Tetranychus, the so-called Red Spider. 

 These cases must of course be carefully dis- 

 tinguished from the normal copper-brown of 

 certain varieties of Beech, Beet, Coleus, etc. 



Silver-leaf. The leaves of Plum, Apple, and 

 other fruit trees often obtain a peculiar silvery 

 appearance in hot summers, the cause of which 

 is unknown. 



Discolorations in the form of confluent yellow 

 and orange patches, etc., resembling variegations, 

 are not infrequently due to the ravages of Red 

 Spider and mites e.g. on Kidney Beans. 



Sun-spots. Yellow spots, which may turn brown 

 or black according to the species of plant affected 

 and the intensity of the action, are often caused 

 by the focussing of the solar rays by lens-like 

 thickenings due to inequalities in the glass of green- 

 houses, or by drops of water on them or on other 

 leaves, e.g. Palms, Dracaena, etc. The action is that 

 of a burning glass, and extends throughout the leaf- 

 tissues. Young grapes, etc., may also be injured 

 in this w^y. Water-drops on the glass can only 

 act long enough to produce such injuries if the 

 atmosphere is saturated. The old idea that a 

 drop on a leaf can thus focus the sun's rays into 

 the tissues beneath is not tenable. 



Here again we see that the disease-agencies 

 concerned in producing the symptoms described 

 in this chapter, agree for the most part in so far 

 that the principal effect is generally the disturbance 

 of chlorophyll action in the spots or flecks on 



