NATURE OF DISEASE. 125 



slowly fill with similarly dark-coloured, impervious 

 masses of materials termed " wound-gum," the 

 nature of which is obscure, but which slowly under- 

 goes further changes into resin-like substances. 



The exposure of wood by a wound results also- 

 in another mode of stopping up the vessels and 

 so hindering the access of air, loss of water, etc., 

 for the living cells of the medullary rays and 

 wood-parenchyma grow^ into the lumina of the 

 larger vessels through the pits, forming thyloses,. 

 again a phenomenon met with in heart-wood. 

 In Conifers the stoppage of the lumina is in- 

 creased by deposition of resin, which also soaks 

 into the cell-walls and the wounded wood becomes 

 semi-translucent owing to the infiltration. 



Every living cell in an active condition is 

 irritable, and one of the commonest physiological 

 reactions of growing tissues is that of responding 

 to the touch of a resistant body, as is vividly shown 

 by the movements of the Sensitive plant, Dionaca, 

 etc., and by those of tendrils, growing root 

 tips, etc., on careful observation. We have reason 

 for stating that if a minute insect, too feeble to 

 pierce the cuticle, cling on to one side of the dome- 

 shaped growing point of any shoot, the irritation 

 of contact of its claws, hairs, etc., would at once 

 cause the protoplasm of the delicate cells to 

 respond by some abnormal behaviour ; and, as 

 matter of experiment, Darwin showed long ago 

 that if a minute piece of glass or other hard body 

 is kept in contact with one side of the tip of a 

 root, the growth on the side in contact is interfered 



