DIV. II 



PHYSIOLOGY 



335 



of the annulus which leads to the opening of the sporangium and the 

 shedding of the spores. 

 With further loss of 

 water the contained 

 water ultimately tears 

 apart from the wall, an 

 air-filled space appears, 

 and the cells of the 

 annulus resume their 

 original form. Since 

 this occurs suddenly, the 

 majority of the spores 

 are forcibly thrown out, 

 as the sporangium again 

 closes. The sporangia of 

 other Vascular Crypto- 

 gams and the walls of FlG 2 76. 1. Sporangium of Polypodium falcatum. (After CAMP- 

 polleil-SaCS afford in their BELL.) 2. Cells of annulus in original position. 3. After 



orjeninp- other examnles partial eva P ration of the water fillin g them 0") > the "Pi** 



cell wall (o) is curved in, while the lower \(u) retains its 

 Of Cohesion-mechanisms. original length. (2, 3 after NOLL.) 



Many hygroscopic 



curvatures also depend on the co-operation of movements depending 

 on imbibition and on cohesion. 



B. Movements of Curvature in the Living Plant 



As in the case of plants which exhibit active locomotion, the 

 phenomena of movement in attached plants may occur when all 

 the general conditions of vital phenomena are present, but sometimes 

 only when a particular factor (stimulus) is acting. The latter deter- 

 mines either the amount of the curvature only or its direction also. 

 Movements which take place without such specific external stimuli 

 are termed AUTONOHIC, while the others are termed INDUCED or 

 PARATONIC movements. 



1 . Autonomie Movements of Curvature 



As stated above, a sufficient intensity of the external factors with 

 which life is associated (p. 218) is sufficient to call forth these move- 

 ments. Beyond a certain minimum and maximum a condition of 

 rigor in which the plant is motionless occurs. Thus, states of rigor 

 due to heat, cold, darkness, dryness, etc., are known. 



Thus also the growth of the shoot or root in a straight line (with 

 the characteristic grand period of growth, dependent, as has been 

 shown, wholly on internal causes) is an autonomic movement. A 

 number of growth curvatures or nutations are associated with this 



