498 TRANSFORMATION OF ENERGY 



of the incurving. Apart from that, the curvature induced by chemical stimuli 

 corresponds exactly to that induced by mechanical stimuli. Darwin proved 

 that many substances, varied in their character, acted as stimuli, when solu- 

 tions of these were placed on the leaf of Drosera. Among these he found that many 

 were neither useful nor yet injurious to the leaf, while, on the other hand, well- 

 known poisons, such as corrosive sublimate, and even nutrients, like many 

 ammonium salts or phosphates, are readily absorbed, or like proteid and pro- 

 teinaceous animal compounds, were first digested by the secretion given off by 

 the glands. Among the substances which were observed to be indifferent from 

 the nutritive standpoint, distilled water may be mentioned more especially, 

 whose value biologically as a stimulant is, according to Correns (1896 b), most 

 perplexing. It must be especially noted that in Correns's experiments the 

 chemically purest water possible was employed ; curvature is not induced if 

 traces of soluble substances be present. Natural water is not a stimulant and 

 that because the fact that it contains lime inhibits the sensitivity of Drosera as 

 do such anaesthetics as ether, chloroform, &c. (compare p. 98). 



Let us now glance at the curvatures induced in the tentacles by transmitted 

 stimuli. Very little is known as to the transmission of the stimulus itself ; it has 

 not been determined whether the agent in transmission is the parenchyma of the 

 leaf (Darwin, 1876 b) or the vascular bundle (Batalin, 1877). So far as we 

 know the stimuli are transmitted exclusively from the tentacles in the centre 

 to those on the outside and not vice versa. The impulse thus transmitted acts 

 directly on the motile zone from beneath ; that it need not first of all be trans- 

 ferred to the apex may be concluded from the fact that decapitated tentacles 

 (Darwin, 1876 b, p. 219) react to a transmitted stimulus, although they are not 

 directly sensitive. This fact, in addition to its general significance, has a special 

 interest, inasmuch as it enables us to interpret a certain phenomenon seen in the 

 sap of the tentacular cells of Drosera which has been spoken of as ' protoplasmic 

 aggregation '. If the apex of a tentacle be stimulated its cells exhibit certain 

 peculiar alterations. Protoplasmic movements first of all take place (De Vries, 

 1886), and the single vacuole breaks up into a large number of small ones. 

 These proceed to contract by apparently ejecting some of their contents, e.g. 

 sugars and organic acids. These vacuoles, surrounded by their protoplasmic 

 membranes, lie in the extruded cell-sap ; the vacuoles contain a red colouring- 

 matter and hence the whole process may be readily followed. The aggregation 

 progresses from the stimulated gland down the tentacle from cell to cell and 

 appears later on in the other tentacles which have been indirectly stimulated. 

 We might interpret this progressive aggregation as the visible expression of 

 the transmission of the stimulus. More accurate investigation, however, reveals 

 the fact that the aggregation appearing in the indirectly stimulated tentacles 

 does not commence simultaneously with the curving but only when the glands 

 begin to secrete. Further, in the indirectly stimulated tentacles the aggregation 

 progresses from above downwards not from below upwards, i. e. in the path 

 of the stimulus. Hence it is obvious that aggregation is a process closely 

 connected with secretion and not with curvature, and we further know that 

 it is associated with very remarkable changes in the nuclei of the gland-cells 

 during secretion (Rosenberg, 1899). It is also worthy of note that, according 

 to Darwin (p. 220), much feebler aggregations are exhibited by decapitated 

 tentacles. 



Apart from the fact that when the tentacle is directly stimulated the 

 stimulus is transmitted only from above downwards to the motile zone and in 

 indirect stimulation from below upwards, there is another and more important 

 difference between the two kinds of stimulation. In the former case it is 

 always a predetermined side of the tentacle (the outside) that becomes convex, 

 while in the latter case the course of the resulting curvature is determined by the 

 direction from which the stimulus comes (p. 497). Either flank of the tentacle 



