THE BLOOMING OF AN UNUSUAL ORCHID. 



79 



travelling from cell to cell until the motor tissues of the hinge are 

 reached. 



There is no development at the hinge of a distinct cushion or 

 pulvinus such as we find at the base of the leaf in the Sensitive 

 Plant, for the lip is of a very light body and easily moved about. 

 The mechanism of the hinge depends for its operation on the smell- 

 ing powers of its thin-walled tissues and their capacity for sudden 

 release of watery contents with consequent contraction of the 

 elastic membranes. To make the mechanism clear we may fix 

 our attention upon a single cell (Fig. 14, B). Suppose the cell 



A 



--CW 



Fig. 14. Masdcvallia niuscosa: A. Side sectional veiw of lip and col- 

 umn in open and closed positions, a, anther; s, stigma; c, column; h, 

 hinge ; cr. crest of lip : p, petal ; c, point of insects' exit. — After Oliver. 



B. Diagrammatic section of cell in the motor region, pr. protoplasmic 

 layer ; cs. cell sap ; cxi', cell wall. 



lying near the upper surface of the hinge to be flaccid, like the 

 other cells in the same vicinity. The lip is now in its raised posi- 

 tion, the hino;e bent. The selected cell is not filled with water to 

 its full capacitv. Its walls are under little or no tension. When 

 the time comes for the lip to be moved downward, the cell begins 

 to absorb water. This happens when the cell begins to produce 

 in the protoplasm soluble substances diffusing into the cell-sap, 

 which are unable to escape through the finer-textured protoplasmic 

 laver next the wall. These confined solubles exert an expansive 

 force such as gases exert in a balloon. Consequently the whole 



