THE COMPARATIVE HISTOLOGY AND IRRITABILITY 



OF SENSITIVE PLANTS 



BY 



D. Walter Steckbeck, M. A., Ph. D. 

 With plates LVIII— LXV. 



Thesis presented to the Faculty of the Graduate School 



in partial fulfilment of the requirements for 



the Degree of Doctor of Philosophy 



Introduction 



The term Sensitive Plants was applied by the earlier botanists to 

 certain forms that exhibit the remarkable phenomenon of "closing up 

 on the approach of man, as if they felt or in some way became aware 

 of his approach. " The application of the term should not be restricted 

 to comparatively few plants, for all plants— all living organisms— are 

 sensitive in that they respond to and are influenced in a variety of ways 

 by environal factors. The marked difference between "sensitives'" 

 and "non-sensitives" lies in the fact that the former have certain 

 mechanisms which enable them to respond relatively quickly to stimuli. 

 Such response usually results in a change of position of the plant or of 

 certain parts of it, and such parts are said to be irrito-contractile. The 

 degree of response, and the duration of the change of position, depend 

 on the character of the stimulus and the time during which it acts. 



This investigation is restricted to a study of some of the sensitive 

 plants— those that have the special mechanisms— (pulvini) referred to 

 above. The leaves of these plants are of especial interest from the 

 irrito-contractile relation. A study of these, as well as the stems, was 

 taken up with a view of ascertaining whether any structural details 

 might aid in explaining the sensitivity of these plants, and whether 

 there is a correlation between structural peculiarities and relative sen- 

 sitivity. 



The movements of leaves in response to various stimuli is observed 

 in many plants. This phenomenon has been studied since the time of 

 Pliny. The most sensitive movements, and those earliest observed are 

 the day and night movements. Sensitive leaves at night occupy posi- 

 tions other than those which they occupy by day, exhibiting a closing 

 and an opening movement. Such changes of position are known as 



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