536 MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 



cells, and causes them to discharge a portion of their water into the adjacent spaces. 

 By this means the turgescence in this part of the cushion is very much diminished, 

 the tissue becomes flaccid, and in proportion as this flaccidity obtains the turgidity 

 in the tissues of the opposite half of the leaf -cushion increases. It seems that a 

 portion of the water given up by the stimulated protoplasm is forced into the 

 opposite tissue, and that thus the turgidity there is augmented. Such a contrast in 

 the turgidity of the two halves of the pulvinus cannot be without influence on the 

 strand of vascular bundles lying in its centre; it becomes bent in the direction of 

 the diminished turgidity, and the leaflet, whose midrib is formed by a continuation 

 of the said vascular bundle-strand, follows this movement. 



In nature, of course, stimulation of the protoplasm by contact of a solid body 

 only occurs exceptionally. There the process described above is brought about by 

 currents of air, and principally by falling rain-drops. Few phenomena have such a 

 peculiar appearance as the movements which occur in the foliage of the already 

 mentioned Oxalis sensitiva when rain comes on. Not only do the leaflets on which 

 the first rain-drops fall, fold together in a downward direction, but all the neigh- 

 bouring ones perform the same movement, although they have not themselves been 

 shaken by the impact of the falling drops, and one is involuntarily reminded of the 

 children's game in which sloped cards are placed behind one another lengthwise in 

 a long series, and the fall of the outermost card, produced by the touch of a finger, 

 causes in a moment the collapse of all the others. But it is not enough that the 

 opposite leaflets, until now flatly outspread, are depressed by the shaking. The 

 movement is continued to the common leaf-stalk bearing the numerous pinnae. 

 This also bends towards the ground, and hangs down apparently prostrated, in 

 consequence of the alteration of turgidity in the pulvinus at its base. The rain- 

 drops now slide over the bent leaf -stalk, whose point is turned towards the ground, 

 and down over the depressed leaflets, and not a drop remains behind on their 

 delicate surfaces. 



The transmission of the stimulus, at first received only by a single leaflet, to the 

 neighbouring leaflets and common leaf-stalk, and finally even to the whole plant, 

 reminds one strongly of the like process in the leaves of the Sundew and of the 

 Venus's Fly-trap. It also recalls the transmission of the stimulus in the protoplasm 

 of lower animals, and is indeed to be explained in a similar manner. Probably the 

 protoplasmic masses of the sensitive groups of cells in all pulvini are connected 

 together by continuous delicate protoplasmic threads penetrating the cell-walls, and 

 the molecular disturbance of the protoplasm, produced by the stimulus, although at 

 first it comprehends only a single cell, is transmitted like an electric current in 

 telegraph wires over the masses of protoplasm, strung together in close connection, 

 and linked by the delicate plasma-threads; thus the same phenomenon is produced 

 in all, viz. contraction of the cells and a forcing out of cell-sap into the intercellular 

 spaces. 



The other sensitive plants behave like the above-described Oxalis sensitiva, 

 except that there is a difference in the direction in which the leaves fold together. 



