550 Dynamic Theory. 



means of communication, and the passage of an influence from the one 



to the other. 



The ends of twining stems, such as hop, morning-glory and bean, 

 sweep around any object suitable for them to cling to, making a turn 

 every few hours. The touch stimulus is the naturally exciting cause of 

 this, and its effects are at, and a little above, the point of contact. 

 Touch stimuli also cause the twining of the tendrils of climbing plants. 

 But in the case of some varieties of the Virginia creeper, the touch or- 

 gan takes the form of a disc or button, and sticks like a sucker to the 

 side of a house or tree. Movements of these plants are also compli- 

 cated by the influence of light, the tendrils traveling much faster in the 

 part of the revolution in which they are moving toward the light. The 

 tendency to twine is, to a certain extent, inherited by some of these 

 plants, because they, in some cases, perform their revolutions before 

 they reach a proper support. 



Darwin names 86 genera, belonging to many different families of 

 plants, which go to sleep at night. This they do by some particular dis- 

 position of the leaves or leaflets, some turning them down in a vertical 

 position, others raising them upright. The Sensitive Plant turns one on 

 top of another. This movement is, to some extent, hereditary. The 

 Sensitive Plant closes up some time before sunset, and opens again be- 

 fore it is fully light. Yet without doubt the habit was originally de- 

 veloped by the action of light. There are some species which sleep 

 after a bright day, but not if the day has been dark. Still, in most of 

 them, the habits are fixed, so that the preparation for night occurs with- 

 out regard to the change in the amount of light. Many of these plants, 

 especially the sensitive plant, will, upon being touched or agitated, fold 

 up their leaves as in sleep. These movements of the leaflets are due to 

 a change which takes place in their little stalks or petioles, and evi- 

 dently a stimulus passes through the cells of the leaflet from the point 

 where it is agitated to the petiole. The petioles of the plants which 

 make these movements are differentiated in a peculiar manner, as shown 

 in fig. 268. This differentiated tract is called a pulvinus (ridge). The 

 cells in this tract are smaller than the ordinar}- cells of the leaf and the 

 rest of the petiole, their development being arrested before they are 

 grown, and they possess little or no chlorophyl. The more sensitive the 

 plant, the greater the differentiation of the cells, and the less chlorophyl 

 do they have. The effect of stimulation is to expand or contract alter- 

 nately, first one side and then another of the pulvinus, and to cause an 

 accession of sap and a turgescence and erection of the tissues ; a pro- 

 cess which appears to be analogous to the action of animal erectile tis- 

 sues. But the point to be especially noted is the transfer, through the 

 cells of the leaf, of the nervous stimulation necessary to set up this ac- 

 tion. We have, in fact, a nervous current without nerves. 



