^b^The Responses 

 of Higher Plants 



THE responses of higher plants are gener- 

 ally slow, subtle, and varied. Nonetheless they 

 are most important to the survival of the 

 species. Plant responses include not only 

 growth and turgor movements (see below), 

 but also flowering and fruiting responses, 

 the germination of seeds and pollen, and a 

 number of other less obvious responses to 

 the changing conditions of the environment. 



TURGOR MOVEMENTS VS. GROWTH 

 MOVEMENTS 



Among higher plants two general types of 

 movement can be distinguished. Growth 

 movements — such as the bending of stems 

 toward light — are so slow that an hour or 

 more is required to perceive a change, and 

 once a growth movement has occurred, it 

 usually is not reversed. Turgor movements, 

 in contrast, are relatively rapid and revers- 

 ible — like the drooping of leaves during a 

 rainfall. Such a difference in tempo and 

 reversibility indicates a difference in the 



basic mechanism of these responses. Growth 

 movements, in fact, depend upon an unequal 

 rate of growth in the cells on opposite sides 

 of the bending part; but turgor move- 

 ments are effected by osmotic changes 

 occurring in specialized effector cells (see 

 below). 



TURGOR MOVEMENT 



The rapid drooping of the leaves of the 

 sensitive plant (Mimosa pudica) is an out- 

 standing example of a turgor movement 

 (Fig. 14-1). The normal position of the leaves 

 of the plant is more or less horizontal; but if 

 one of the leaves is touched, even lightly, all 

 the leaflets begin to fold up, and within 2 or 

 3 seconds the whole leaf droops to a marked 

 degree. Or if a leaf is struck less gently, not 

 only does the stimulated leaf fold and droop, 

 but a wave of folding and drooping spreads 

 from the stimulated leaf to all neighboring 

 leaves (Fig. 14-1). This response of the mi- 

 mosa is strictly reversible, and after about a 



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