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 involved, 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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