VARIATION OF POLAR REACTION 267 



physiological factors. Among the most important of these 

 may be mentioned the influence of age and of season. 

 Experiments on the effect of age in the modification of 

 response will first be described. 



The Effect of Age 



It is impossible to dissociate from the consideration of 

 the age of a given leaf its past history as regards the stimulus 

 of sunlight. If we examine a plant we find that the youngest 

 leaf is quite green, not having as yet been long exposed to 

 the action of light. The next below it will be older, and 

 owing to the longer action of the sunlight, reddish-brown in 

 colour. The leaves lower down will be older again and still 

 more russet in tint. In this way the sequence of the leaves 

 in point of age, from above downwards, corresponds to the 

 other sequence of duration of exposure to stimulus of light. 



It will be remembered that we may broadly classify the 

 condition of a tissue under three different phases : first, the 

 pre-optimum or sub-tonic state ; second, the optimum ; 

 and third, the post-optimum condition, in which we see 

 an approach towards the condition of fatigue. Moderate 

 stimulation, as we have seen, will carry a tissue out of the 

 pre-optimum towards the optimum condition, with con- 

 sequent enhancement of excitability. Excessive or too- 

 prolonged stimulation, on the other hand, will carry it 

 to the post-optimum condition with the characteristic 

 depression of its excitability. 



From this point of view alone, then, we might expect that 

 the uppermost or youngest leaf of Mimosa would be in the 

 pre-optimum and therefore less sensitive condition ; that 

 the sensitiveness of the leaves should attain a maximum as 

 we descend lower in the plant ; and that after this has been 

 reached, continuing to descend, the excitability of the 

 different leaves will be progressively decreased. Represent- 

 ing these gradations by means of a curve, there would be at 

 first an ascent, then a climax, and after this a sharp turn and 



