THE PLANT WORLD 61 



would be like the bear living by sucking its own paws — there would soon 

 be nothing but bacteria left to feed on one another. In our own bodies 

 we have the white blood-corpuscles whose function it is to destroy any 

 bacteria that may find an entrance into the blood. In some diseases 

 infected spots are shut off from the rest of the body by layers of resistant 

 tissue that keep the "germs" from getting into the general circulation, 

 and in time causes their destruction from lack of food. Plants do not 

 have white corpuscles, but it is certain that they have some means of 

 resisting such attacks. 



Perhaps the simplest way of keeping out bacteria would be the drying 

 of the tissues around the wound, and we find that this takes place fre- 

 quently. The hard, dry cell walls do not easily yield nourishment to the 

 "germs," as we know from the length of time seasoned timber lasts. 

 Cellulose, which is the principal constituent of the cell-walls, is closely 

 related to sugar, starch, glucose, and a number of gums, but it is much 

 more resistant to the action of chemicals, and digestive fluids do not 

 readily dissolve it. When the wound is fresh there is a greater chance 

 that the cellulose maj^ be eaten away. After drying it resists the attacks 

 long enough to give the plant an opportunity to strengthen its defenses. 

 Among many cryptogamous plants this is the sole method of protection, 

 while only a few phanerogams depend on it alone. As a rule, they 

 produce a layer of " wound-cork " that cuts off the injured spot from the 

 underlying parts of the stem. This is nearly a complete cure in the case 

 of the more tender parts. Woody stems form a " callus " by the growth 

 of the surrounding cells that afterwards form a corky layer. The new 

 wood gradually spreads over the wound until the edges meet and 

 coalesce. Outwardly it seems as if there had been no harm done, but 

 the injured cells inside remain brown and dead, and can be seen until 

 the decay of the tree, by cutting into the wood. 



When a thin section of the ' ' wound wood ' ' is examined under the 

 microscope it is seen to be made up of nearly cubical cells that are quite 

 unlike the usual elongated cells of normal wood. As the tree increases 

 in diameter these wound cells become more and more like those of 

 uninjured wood. 



Many plants possess strongly-smelling ' ' ethereal oils ' ' that play a 

 part in warding off enemies that might otherwise use them for food. 

 These substances are often contained in special glands or receptacles. 

 The different gums and resins occur in similar canals and receptacles 

 throughout the plant. One of their principal uses seems to be to flow 

 out and cover up wounds, and in this way to prevent the entrance of 

 injurious fungi or bacteria. Cherry-tree gum is familiar to all, and 

 the resin on pines and the related trees will keep away almost anything, 

 including bacteria, but a small boy with climbing proclivities. 



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