STIMULATION AND ITS RESULTS 405 
The various positions which are assumed by the different 
subaerial organs of plants are evidently those in which 
they can react most advantageously with their environment. 
It must be borne in mind, however, that in every case 
during natural life the plant is receiving coincidently several 
kinds of stimulation, the effect of some being not infre- 
quently antagonistic to that of others. It is not easy to 
discriminate between these, nor to say how the influence of 
each helps to determine the resultant response. This is the 
more difficult, as not only the stimuli themselves, but their 
relative potencies’ differ continually. 
Another form of stimulation differs from those we have 
discussed, in that its effect becomes evident in the cells 
actually stimulated. This is the stimulus of internal pressure. 
When the central cylinder ofa stem begins to show 
secondary thickening a strong pressure from the new 
vascular tissue sets up considerable tension in the outer 
layers of the cortex, and tends to rupture the epidermis. 
This strain is quickly followed by the appearance of a 
merismatic layer, the cork phellogen, which increases the 
bulk of the cortex in the affected area, and produces as well 
a layer of cork. 
A similar cause leads to the appearance of the inter- 
fascicular cambium acrogs the medullary rays. The 
cambium of the still isolated bundles beginning to form 
new wood and bast, a strain of the medullary ray tissue 
lying over against the new products is the result. This 
strain is responded to by the gradual formation of meristem 
—the interfascicular cambium—which slowly extends 
across the ray from one bundle to the next. 
Another form of it is seen in the response any particular 
cell makes to the increase of its own turgor. 
The infliction of a wound is always followed by an 
increased growth of the injured tissues, or those near them. 
