BOOK I. VEGETABLE PATHOLOGY. 191 



from animals with sufficient accuracy by means of the trial of burning ; as animal sub- 

 stances in a state of ignition exhale a strong and phosphoric odor, which vegetable sub- 

 stances do not. 



CHAP. IX. 

 Vegetable Pathology, or the Diseases and Casualties of Vegetable Life. 



859. As plants are, like animals, organised and living beings, they are, like animals 

 also, liable to such accidental injuries and disorders as may aftect the health and vigor, 

 or occasion the death of the individual. These are wounds, accidents, diseases, and 

 natural decay. 



SECT. I. Wounds and Occidents. 



860. A wound is a forcible separation of the solid parts of the plant effected by means 

 of some external cause, intentional or accidental. 



861. Incisions are sometimes necessary to the health of the tree, in the same manner perhaps as 

 bleeding is necessary to the health of the animal. The trunk of the plum and cherry-tree seldom expand 

 freely till a longitudinal incision has been made in the bark ; and hence this operation is often practised 

 by gardeners. If the incision affects the epidermis only it heals up without leaving any scar ; if it pene- 

 trates into the interior of the bark, it heals up only by means of leaving a scar ; if it penetrates into the 

 wood, the wound in the wood itself never heals up completely, but new wood and bark are formed above 

 it as before. 



this the sap flows copiously ; and though a number of holes is often bored in the same trunk, the health of 

 the tree is not very materially affected. For trees will continue to thrive though subjected to this oper- 

 ation for many successive years ; and the hole, if not very large, will close up again like the deep incision, 

 not by the union of the broken fibres of the wood, but by the formation of new bark and wood projecting 

 beyond the edge of the orifice, and finally shutting it up altogether. 



863. Girdling is an operation to which trees in North America are often subjected when the fanner 

 wishes to clear his land of timber. It consists in making parallel and horizontal incisions with an axe into 

 the trunk of a tree, and carrying them quite round the stem so as to penetrate through the alburnum, 

 and then to scoop out the intervening portion. If this operation is performed early in the spring, and be- 

 fore the commencement of the bleeding season, the tree rarely survives it ; though some trees that are pe- 

 culiarly tenacious of life, such as acer saccharinum and nyssa integrifolia, have been known to survive 

 it a considerable length of time. 



864. Fracture. If a tree is bent so as to fracture part only of the cortical and woody fibres, and the stem 

 or brancli but small, the parts will again unite by being put back into their natural position, and well 

 propped up. Especially cure may be expected to succeed if the fracture happens in the spring ; but it will 

 not succeed if the fracture is accompanied with contusion, or if the stem or branch is large j and even where 

 it succeeds the woody fibres do not contribute to the union, but the granular and herbaceous substance 

 only which exudes from between the wood and liber, insinuating itself into all interstices and finally be- 

 coming indurated into wood. 



865. Pruning. Wounds are necessarily inflicted by the gardener or forester in the pruning or lopping 

 off the superfluous branches, but this is seldom attended with any bad effects to the health of the tree, if 

 done by a skilful practitioner: indeed no further art is required merely for the protection of the tree be- 

 yond that of cutting the branch through in a sloping direction so as to prevent the rain from lodging. In 

 this case the wound soon closes up by the induration of the exposed surface of the section, and by the pro- 

 trusion of a granular substance, forming a sort of circular lip between the wood and bark ; and hence the 

 branch is never elongated by the growth of the same vessels that have been cut, but by the protrusion of 

 new buds near the point of section. 



866. Grafting. In the operation of grafting there is a wound both of the stock and graft; which are 

 united, not by the immediate adhesion of the surfaces of the two sections, but by means of a granular 

 and herbaceous substance exuding from between the wood and bark, and insinuating itself as a sort of 

 cement into all open spaces : new wood is finally formed within it, and the union is complete. 



867. Felling is the operation of cutting down trees close to the ground, which certain species will survive, 

 if the stump is protected from the injuries of animals, and the root fresh and vigorous. In this case the 

 fibres of the wood are never again regenerated, but a lip is formed as in the case of pruning ; and buds, 

 that spring up into new shoots, are protruded near the section ; so that from the old shoot, ten, twelve, or 

 even twenty new stems may issue according to its size and vigor. The stools of the oak and ash-tree 

 will furnish good examples ; but there are some trees, such as the fir, that never send out any shoots after 

 the operation of felling. 



868. If buds are destroyed in the course of the winter, or in the early part of the spring, -many plants 

 will again generate new buds that will develope their parts as the others would have done, except that they 

 never contain blossom or fruit. Du Hamel thought these buds sprang from pre-organised germs which 

 he conceived to be dispersed throughout the whole of the plant ; but Knight thinks he has discovered the 

 true source of the regeneration of buds, in the proper juice that is lodged in the alburnum. Buds thus re- 

 generated never contain or produce either flower or fruit. Perhaps because the fruit-bud requires more 

 time to develope its parts, or a peculiar and higher degree of elaboration ; and that this hasty production 

 is only the effect of a great effort of the vital principle for the preservation of the individual, and one of 

 those wonderful resources to which nature always knows how to resort when the vital principle is in dan- 

 ger. But though such buds do not produce flowers directly, as in the case of plants that bear their blos- 

 soms on last year's wood ; yet they often produce young shoots which produce blossoms and fruit the 

 same season, as in the case of cutting down an old vine, or pruning the rose. 



869. Sometimes the leaves of a tree are destroyed partially or totally as soon as they are protruded from 

 the bud, whether by the depredations of caterpillars or other insects, or by the browsing of cattle. But 

 if the injury is done early in the spring, new leaves will be again protruded without subsequent shoots. 

 Some trees will bear to be stripped even more than once in a season, as is the case with the mulberry-tree, 

 which they cultivate in the south of France and Italy for the purpose of feeding the silk-worm. But if it is 

 stripped more than once in the season it requires now and then a year's rest. 



870. The decortication of a tree, or the stripping it of its bark, may be either intentional or accidental, 

 partial or total. If it is partial, and affects the epidermis only, then it is again regenerated, as in the case 

 of slight incision, without leaving any scar But if the epidermis of the petal, leaf, or fruit, is destroyed, 

 it is not again regenerated, nor is the wound healed up, except by means of a scar. Such is the case also 



