I/O The Sense-Organs of Plants. [Sess. 



In the case of ordinary leaves without motile pulvini, 

 which depend on growth movements for their diaheliotropic 

 orientation, variations in light- direction are perceived by the 

 leaf-blade, while the stalk is the motor organ. A few years 

 ago Haberlandt (' Lichtsinnesorgane der Laubblatter, 1905) 

 made the interesting discovery that certain epidermal cells of 

 the upper surface of the leaf acted as rudimentary eyes or 

 ocelli. Vertical light-rays passing through the plano-con- 

 vex outer wall of these cells into the cell interior undergo 

 refraction, and a spot of light is thrown on the cytoplasm 

 lining the basal wall of the cell. When the light becomes 

 oblique the light-spot shifts a little to the side. This varia- 

 tion in position of the light-spot acts as a stimulus, the 

 response to which is a growth movement which brings the 

 leaf into the position of heliotropic equilibrium when 

 the light-spots are once more central. Further research 

 has shown that the ocelli are not confined to leaves, but also 

 occur on petioles, petals, and on some stems. The view of 

 Haberlandt has been strongly criticised, and weighty objec- 

 tions urged against it. These are fully marshalled and dis- 

 cussed in an exhaustive paper recently published by Wager 

 (*Ann. of Botany,' 1909, xxiii. 459). There is no doubt as 

 to the existence of the light-spots, for they can be seen and 

 photographed from micro - preparations of the epidermis. 

 Further, that the plano-convex walls of the ocelli act as lenses, 

 and are very good ones too, is proved by the fact that very 

 excellent photographs have been taken by means of them. 



Thigmotropism and Seismonic Irritability. — Pfeffer 

 ('Physiology of Plants,' iii. 63) has pointed out that 

 there are two kinds of contact irritability — viz., seismonic 

 irritability or responsiveness to any kind of mechanical shock, 

 such as impact of rain-drops, earth-vibrations, wind, and the 

 true sense of "touch — thigmotropic irritability — possessed 

 by special organs, such as tendrils, tentacles of Drosera, &c. 

 In the latter the sensation of contact is produced under the 

 same conditions as in animals. The stimulus is not mere 

 pressure, but friction against a solid body applied so as to 

 produce deformation in the outer epidermal wall, and thereby 

 affecting the irritable protoplasm. The contact must therefore 

 be directly on the cell-wall. 



