254 DISEASE IN PLANTS. 



Leaf-rolling. The leaves of Beeches, Poplars, 

 Limes, and many other plants, instead of opening 

 out flat, are often rolled in from the margins, or 

 from the apex, by various species of Pliytoptus, 

 Cecidouiyia, or other insects, which puncture or 

 irritate the epidermis in the young stages and so 

 arrest its expansion in proportion to the other 

 tissues. According as the lower or upper surface 

 is attacked the rolling is from the morphologically 

 upper surface downwards, or vice versa. Very 

 often the mesophyll is somewhat thickened where 

 rolled and Erineiini-\\V.& hairs may be developed 

 e.g. Lime. Many caterpillars also roll leaves, 

 drawing the margins inward to form shelters e.g. 

 Tortrix viridana, the Oak leaf-roller. Certain 

 beetles Rhynchitis also roll up several leaves to 

 form a shelter in which the eggs are laid. 



Webs are formed among the mutilated leaves 

 of Apples by the caterpillars of Hyponomeuta. 



It must be borne in mind that instances can be 

 found of teratological change of every organ in 

 the plant e.g. stamens transformed into carpels or 

 into petals ; anthers partly polliniferous and partly 

 ovuliferous ; ovules producing pollen in their 

 interior, and so on, being simply a few startling 

 examples of what may happen. Such abnormali- 

 ties are frequently regarded as evidence of internal 

 causes of disease, and this may be true in given 

 cases ; in a number of cases investigated, however, 

 it has been shown that external agents of very 

 definite nature bring about just such deformations 

 as those sometimes cited as examples of teratology 



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