412 MOVEMENTS. 



the same plant which are allowed to take the protective position, 

 escape. 



1067. Sleep-movements of floral organs. These are, in general, 

 dependent, as Pfeffer has clearly shown, upon the alternate, 

 growth of the opposed surfaces. For instance in a crocus, the 

 greater growth of the inner surface of the parts of the perianth 

 will bring about an opening of the flower, whereas the greater 

 growth of the outer surface will effect a closing. 



Pfeffer's method of investigation is capable of application, pro- 

 vided one has a microscope which admits of being held with its 

 tube horizontal. A perianth leaf is carefully detached without 

 too much violence from the flower, and immediately placed in a 

 small tube containing water, so that the expanded part may be 

 brought within the field of the microscope. If fine lines are 

 measured off upon its inner and outer surfaces in India ink, 

 their gradually increasing distance from each other can be 

 watched to good advantage. It can then be clearly seen that 

 when the part curves outward it is owing to an increased growth 

 upon the inner surface, and vice versa. That there is an ante- 

 cedent turgescence is very likelv, as has been repeatedly pointed 

 out by De Vries and others. It is probable also that in a few 

 cases the opening and closing are due to a temporary turges- 

 cence unaccompanied Iry much growth. 



Changes in illumination and in temperature are sufficient to 

 effect the alternations of growth and of turgescence in delicately 

 constituted parts, where there is a balanced tension existing 

 between the outer and inner tissues. 



1068. Times of opening- and closing- in the open air. Under the 

 ordinary conditions of an equable climate the times of opening 

 and closing of the flowers of a given plant do not vary widely. 

 Hence it is possible to construct a floral clock which shall mark 

 the hours with tolerable regularity. The dial at Upsala, Sweden, 

 suggested by Linnaeus, and that designed for Paris by De Can- 

 dolle, 1 are approximately correct; but in a climate having the 

 sharp and sudden differences of heat and of moisture which 

 characterize eastern North America such floral clocks are not 

 successful. 



1 The following list from De Candolle's Physiologic gives the hours of the 

 opening of certain flowers in Paris: - 



Ipomoea purpurea 2 A. M, 



Calystegia sepium ...... ^ ... 3-4 ----- 



Matricaria suaveolens 4-5 " 



