412 MOVEMENTS. 



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

 escape. 



1067. Sleep-movements of floral organs. Tliese 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 l)ring about an opening of the flower, whereas the greater 

 growth of the outer surface will effect a closing. 



Pfefffer'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 cai-efully 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 fleld 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 clearl}- 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 ver\- likely-, as lias been repeatedly pointed 

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

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

 cence unaccompanied hy much growth. 



Changes in illumination and in temperature are sufficient to 

 effect the alternations of growth and of tnrgescence 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 

 ordinarj- conditions of an equable climate the times of opening 

 and closing of the flowers of a given plant do not varj' widel3-. 

 Hence it is possible to construct a floi'al clock which shall mark 

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

 suggested by Linnaeus, and that designed for Paris liy Ue Can- 

 dolle,^ 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. 



^ The following list from De Candolle's Physiologie gives the hours of the 

 opening of certain flowers in Paris : — 



Ipomoja purpurea 2 A. M. 



Calystcgia sepium 3-4 " 



Matricaria suaveoleus 4-5 " 



