PHYSIOLOGICAL PROCESSES DURING FLOWERING 437 



unequally distributed through the plant and that the nio«t sensi- 

 tive part is the lower surface of the pulvinus, the swollen base 

 of the petiole. The mechanism of the movement consists in a 

 change of turgor in the upper and lower halves of the pulvinus, 

 which supports the leaves (Fig. 137). 



The stimulus suddenly increases the permeability of the 

 protoplasm in the lower half of the pulvinus. The cell wall, 

 which was formerly under considerable tension, becomes fla(;cid 

 since the protoplast no longer presses against it when its 

 permeability to water causes a decrease in volume of the proto- 

 plast. At the same time, part of the cell sap passes out into the 

 intercellular spaces. This results in a rapid decrease of turgor 

 in the lower half of the pulvinus. As the upper half preserves 

 all of its tension, the petiole will bend downward, and the leaf 

 droops. This extrusion of sap from the cells into the intercellular 

 spaces is indicated by the lower half of the pulvinus becoming 

 transparent. If a shock is produced in a leaf that has been cut 

 off close to the stem and left to recover in a humid atmosphere, 

 then it may be observed that the pulvinus curves and that a 

 drop of liquid appears at its cut end. As the reaction passes, the 

 liquid is reabsorbed by the leaf, and the pulvinus again straight- 

 ens out. 



100. Physiological Processes Accompanying Pollination and 

 Fertilization. Germination of Pollen and Growth of the Pollen 

 Tube. Phenomena Accompanying Termination of Blossoming. 

 Shedding of Immature Fruit and Its Causes. — The process of 

 fertilization and the subsequent development have been studied 

 in great detail, both morphologically and cytologically. The 

 physiology of the process, on the other hand, has been very little 

 investigated. Because of its complexity, it seems to be difficult 

 to subject the process to a detailed physiological analysis. Only 

 a brief mention will be made here of the most important achieve- 

 ments in this field. The consideration of fertihzation will be 

 limited to the angiosperms, and the morphological and cytological 

 side of the process will not be touched upon, since they are 

 sufficiently described in textboolvs of plant morphology and 

 cytology. 



Fertilization as such is preceded by pollination, i.e., the transfer 

 of pollen from the anther to the stigma, where the pollen germin- 

 ates. In the germination of pollen, the external coat of the 

 pollen grain (extine) is ruptured in definite places, while its 



