Chapter XII — 191 — Dionaea and Aldrovanda 



sap into the intercellular spaces, such as is well known to take place 

 in the pulvini of Mimosa during movement response, denied that 

 there is any extrusion of water by the cells in the upper moiety of 

 the trap lobe, and Brown denying the contraction of the upper face 

 sees no necessity for such extrusion, but falls back on the expansion 

 of the cells of the lower face. This makes it necessary to find a move- 

 ment of water from a source sufficient for this expansion. The only 

 source considered is the parenchyma of the upper face, in which, since 

 the intercellular spaces are not flooded, water must pass from cell 

 to cell; the movement would then resemble that of geotropism. 

 An acknowledged difficulty is seen in the rapidity of the response 

 which, though often slow enough, is at times and normally so rapid 

 that complete closure is reached in the space of even less than a 

 half second (77 — 6). This difficulty must be faced as also that 

 arising from the attempt to account for the loss of water by some 

 cells (those of the upper face) by changes in the substances present 

 in the cells to less osmotically active ones, thus permitting the water 

 to be drawn off into other cells (those of the lower face) to facili- 

 tate their expansion. How sufficient water can thus be moved to 

 procure the recorded amount of expansion of the lower face, with- 

 out causing a reduction (contraction) of the upper face in even greater 

 amount (since the latter is shorter, if only slightly), is not clear. 



This rapidity of movement seems to demand that there be a con- 

 dition of unstable equilibrium resulting from growth and residing in 

 the trap lobes. Batalin advanced this idea but he was, it is recalled, 

 committed to explain a shortening of the upper face. That tissue 

 tensions do exist may be taken for granted (Darwin), just as they 

 exist in the valves of the fruit of Impatiens. In this plant the ten- 

 sions are held in check by mechanical conditions, namely, the mu- 

 tual adherence of the valves. This disturbed, the valves spring away 

 by immediately curving in the same sense as the lobes of the Dionaea 

 trap. In the latter the lobes maintain their form, unless stimulated, 

 by the opposition of the two epiderms to their contiguous tissues. 

 When stimulated, the balance of forces is upset and curvature imme- 

 diately follows. That is, when stimulation takes place something 

 happens to release the tensions. What this something is we do not 

 yet know. If we might postulate chemical changes in the cell 

 contents from sugar to starch in the upper surface of the trap lobes, 

 it would serve us with a mechanism for changing the tensions, but 

 sufficiently rapid changes are not known. Brown's experiment in 

 which he substituted xylene, in which sugars are insoluble, for water, 

 are suggestive, but not convincing further than showing that ten- 

 sions exist which might be released by such a mechanism. This is 

 in essence the theory put forth by voN Guttenberg (1925) who be- 

 lieves that the movement is caused, not by any reduction of turgor 

 in any tissues whatever, but by the drag of the parenchyma on the 

 two epiderms (upper and lower), the upper being thicker and less 

 extensible than the lower {iS — 5, 6, 10), as Macfarlane also main- 

 tained. That this drag is positive, exerting a pull on the epiderms, 

 is indicated by the fact that if the tissues of the upper epidermis 

 and the contiguous parenchyma are partially robbed of water by 



