334 DISEASES OF PLANTS. 



wood. Boucherie proposed that a solution of pyrolignite of iron 

 should be introduced into trees before being felled, by making perfora- 

 tions at the base of the trunk, and allowing the absorbing power of the 

 cells and vessels to operate. This plan does not appear to have been 

 successful, although reported favourably to the French Academy, and 

 also recommended by Mr. Hyett. 



700. Other diseases in plants owe their origin to insects. Earcockles, 

 purples, or pepper- corn, is a disease affecting especially the grams of 

 wheat. The infected grains become first of a dark green, and ulti- 

 mately of a black colour. They become rounded like a small pepper- 

 corn, but with one or more deep furrows on their surface. The glumes 

 spread open, and the awns become twisted. The blighted grains are 

 full of a moist white cottony matter, which, when moistened, and put 

 under the microscope, is seen to consist of a multitude of minute in- 

 dividuals of the Vibrio triciti, or eel of the wheat. The animalcules 

 deposit their eggs in the ovary, and their young are hatched in eight 

 or ten days. Henslow calculates that 50,000 of the young might be 

 packed in a moderately sized grain of wheat. The Vibrio retains its 

 vitality long. It will remain in a dry state for six or seven years, and 

 when moistened with water will revive. The Wheat-fly, or Cecidomyia 

 tritici, is another destructive insect. It deposits its eggs by means of 

 a very long retractile ovipositor, and is seen abundantly in warm even- 

 ings. The Cecidomyia destructor, or Hessian fly, also causes injury, 

 and is said to be very destructive to wheat in America. These insects 

 are destroyed in numbers by the Ichneumons, which deposit their ova 

 in their bodies. The Apple-tree mussel, or dry-scale, Aspidotus con- 

 chiformis, attacks the bark of Apples, Pears, Plums, Apricots, and 

 Peaches. Many of the Coccus tribe are highly injurious to plants. 

 One of this tribe, in 1843, destroyed the whole orange trees in the island 

 of Fayal, one of the Azores. Many insects cause the rolling up of leaves. 

 Tortricida viridana acts thus on the leaves of the Oak, and various 

 species of Losotgenia do so with other trees. Sacchiphantes abietis is 

 the aphis which causes the leaves of the Spruce-fir to be united together, 

 so as to have the appearance of a cone. 



701. Many insects, called miners, make their way into the interior 

 of leaves, and hollow out tortuous galleries, sometimes causing an 

 alteration in the colour of the leaves. Galls are caused by the attacks 

 of species of Cynips, which are provided with ovipositors, by means of 

 which they pierce the bark or leaves with the view of having a nidus 

 for their ova. These galls are very common on the Oak, and are 

 called oak-apples. Sometimes they have one cavity, at other times 

 they are divided into numerous chambers, each containing a grub, 

 pupo, or perfect fly, according to the season. 'Galls are produced on 

 the twigs, catkins, and leaves of the Oak. The artichoke gall of the 

 Oak depends on an irregular development of a bud, caused by the 



