STONE: CONTACT STIMULATION 477 



sucker growth around stumps, the enclosure of twigs, grass, etc., by 

 mushrooms, and the imbedding of tree guards illustrate stimulated 

 growth responses not generally associated with wounding. When 

 more or less long stubs remain following the destruction of limbs on 

 trees, they act as a stimulus, and large masses of tissue ultimately 

 accumulate around the base of the same. If, however, the stub is 

 removed close to the tree, healing follows, and when the callus unites 

 growth becomes normal. Again the feathery growths, consisting of 

 small twigs which are common on the trunks of elm trees, give rise to 

 the same type of reactions, namely, the formation of excrescence or 

 tuberous growths at their point of junction to the tree. The presence 

 of blocks of wood on trees to prevent the contact of guy wires with 

 the bark often stimulates the flow of tissue, and while there is appar- 

 ently no injury associated with this connection there exists a more 

 or less severe tension or pressure on the bark. The tendency of 

 tissue to grow over even loosely attached tree guards and wire attach- 

 ments, even when it would require but little force to dislocate them, 

 is universal. Signboards placed on trees, although often insecurely 

 attached, are sometimes entirely obliterated by a greatly stimulated 

 callous growth, and tree guards loosely attached to trees often become 

 imbedded in the tissues, when it would require but little exertion on 

 the part of the underlying tissue to dispose of the same. It may be 

 assumed, however, that the presence of nails driven into the wood 

 for the purpose of holding the sign in place would act as a stimulus, 

 but the same reaction follows if the nails are not present and the sign 

 merely held securely by means of wires. Moreover, the growth 

 response here conforms to the sign as a whole rather than to any par- 

 ticular point of attachment by the use of nails, etc. Similar response 

 occurs in the growing of roots around large masses of gravel in the 

 soil or when limbs or roots come in contact with one another, in which 

 case natural grafts may occur. The tissue, however, exterior to that 

 which is in contact is often destroyed in natural grafts, etc., which 

 would have the same effect as wounds; although even here the growth 

 stimulation is much greater than that resulting from mere wounding. 

 In this type of response there are two factors invoK^ed, namely con- 

 tact and growth acceleration. The reaction, however, is local and does 

 not differ materially whether injuries occur or not, as is shown by 

 growth stimulations arising from other causes, such as frost cracks, 

 etc. There are other similar types of reactions apparently differing 

 somewhat in the nature of the response to contact from those enumer- 

 ated, namely, those arising from restricted growth. Contact in such 

 cases occurs, but injury may be absent, although abnormal growth may 

 result. This type seems to be characterized in their manner of re- 

 sponding by a greatly increased osmotic tension of the cells. 



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