5.2.1 Specialized touch responses
Many plants have highly specialized mechanosensory structures capable of remarkably
sensitive and selective mechanical signaling. Carnivorous plants show some of the most
dramatic examples of this class of touch responses. In the Venus fly trap, leaves are mod-
ified into a bilobed trap that closes in less than a second to capture insects for subsequent
digestion (Darwin 1893; Simons 1992). This is a thigmonastic response in that the direc-
tion of the response is inherent to the structure of the organ and is not entrained by the
direction of the touch stimulus. It is thought that the trap closes by rapid growth at the
92 PLANT TROPISMS
Figure 5.1. Plants experience a complex mechanical environment that differs significantly above- and be-
lowground. The root system tends to be supported in the soil to offset the effects of the weight of the organs,
whereas aerial parts of the plant must support their own weight. The effects of incidental contact with animals,
herbivory, cultivation, and obstacles to the current direction of growth are experienced throughout the mature
plant. The effects of soil impedance are limited to roots and the seedling shoot prior to emergence from the
soil. The direct effects of wind and rain are limited to the aerial parts of the plant. The mechanical stimulus
from all these factors has been shown to significantly affect plant form and to have important real-world im-
plications, for example, placing limits on agricultural productivity (Mitchell 1996; Zheng et al. 2000).