STIMULATION AND ITS EESULTS 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 1 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 of a 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 across 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 meristern 

 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. 



