DEHISCENCE OF ANTHERS 555 



treatment with potash, which reagent also causes the cell-walls to 

 swell very considerably. In these circumstances, they exhibit a very 

 well-marked right-handed torsion, which can be readily observed 

 under the microscope; drying conversely produces a left-handed twist. 

 The entire mechanism thus again depends essentially upon the fact, that 

 the tangential imbibition is greatest at right angles to the long axes of 

 the sinistral micellar series, and smallest in the direction parallel to 

 those series. 



The present section may conclude with a brief consideration of the 

 dehiscence-mechanisms of anthers, a difficult problem which has formed 

 the subject of numerous researches, but which still remains partially 

 unsolved. As a general rule, each anther-lobe dehisces by means of 

 a single longitudinal slit, which is common to the two pollen-sacs. In 

 dry air, the two valves or flaps of the anther wall curl away from the 

 slit, and thus allow the pollen to escape freely. If the anther is wetted, 

 the valves at once uncurl and close over the slit. The hypodermal cells 

 of the valves are provided with very characteristic fibrous thickening ; 

 they constitute the so-called fibrous or mechanical layer, which is alto- 

 gether responsible for the movements of the valves, the epidermis taking 

 no active part whatever in that process. According to Steinbrinck, the 

 fibrous thickenings are generally arranged, in each cell, in such a way 

 that " they traverse the whole length of the radial walls, and unite on 

 the inner tangential walls to form a stellate or reticulate framework, 

 a system of parallel bars, or sometimes even a massive plate, while the 

 outer tangential wall always remains entirely unthickened." The fibrous 

 layer therefore possesses two antagonistic sides, consisting respectively 

 of the outer and the inner tangential walls, that differ appreciably as 

 regards their rigidity, one being stiffened by means of special thickenings 

 which are wanting in the other. During the tangential contraction of 

 the fibrous cells which follows upon loss of water, their delicate outer 

 walls will tend to collapse more than their inner walls, since the latter 

 are supported by thickening fibres (Fig. 226); the whole valve will 

 consequently curl outwards in dry air. According to this view, which 

 was first put forward by Leclerc du Sablon, the dehiscence of anthers 

 depends upon a genuine hygroscopic mechanism, the active component 

 of which is represented by the unthickened portions of the radial walls 

 of the fibrous cells. Furkinje, Meyer, Chatin and Schinz, on the other 

 hand, all attribute the movements of anther-valves to the hygroscopic 

 properties of the fibrous thickenings themselves ; this hypothesis has 

 recently been revived by J. M. Schneider, who maintains that, in the 

 case of Tulipa, at any rate, the inner side of each curved thickening 

 fibre is more hygroscopic than the outer side, with the result that the 

 curvature of all the fibres increases on drying. 



