426 



TRANSFORMATION OF ENERGY 



and again remarked that equalizing of tensions is generally brought about after 

 the application of a stimulus, such as contact, shaking, &c., but that they may 

 take place to all appearance of their own accord when the tension has reached 

 a certain amount. To prove this is, however, by no means easy, for, obviously, 

 the nearer the organ approaches to a condition of ripeness, shocks, ever de- 

 creasing in intensity, are sufficient for the purpose, the application of which are 

 scarcely avoidable in the course of the observation. We have yet to discuss 

 the significance of these shocks. 



In the preceding pages we have often spoken of such shocks, and have 

 seen that very frequently, by their means, energies stored up in the organisms 

 are released ; they act as ' stimuli.' In speaking of plant formation, we saw 

 that contact especially might act in this way, and in Lect. XXXVIII we shall 



learn to recognize a whole 

 series of movements which 

 were initiated by contact. The 

 question now before us is, 

 must we group such slinging 

 movements as we have just 

 been discussing alongside the 

 movements resulting from the 

 application of stimuli,such as we 

 have yet to consider,'or are they 

 to be placed in another category, 

 viz. 'autonomous' movements? 

 By autonomous move- 

 ments we mean such as are in- 

 duced by some internal factor 

 and not the result of the appli- 

 cation of an external stimulus 

 (Lect. XLI). From what has 

 been said it is evident that 

 the ejaculatory movements 

 manifested by spores and suc- 

 culent fruits may be autono- 

 mous. When such movements 

 follow a blow or shaking it is 

 obvious that the immediate 

 result consists only in a local 

 increase in tension which would 

 arise spontaneously in the 

 course of further ripening. 

 Seeing that the blow does not provide the force which brings about the 

 ejection we have here to deal with a releasing force only, or a stimulus. This 

 stimulus differs essentially from those usually met with in the plant, so that it 

 is doubtful whether the ejaculatory movements are to be counted as stimulus 

 movements or not. There are, however, slinging movements which are 

 undoubtedly typical stimulus-movements, and in order to show clearly the 

 difference between these two types it will be advisable to discuss one example. 



The slinging movement in question is exemplified in the flower of the 

 orchid Catasetum, the structure of which will be made clear by a study of 

 Fig. 131 (Darwin, 1877). In / the entire flower is shown after removal of five 

 of the perianth leaves ; the one left is the large labellum {I). In the middle 

 stands the column, from the face of which project two horn-like appendages, 

 the so-called antennae {an). Looking at the column in face view (//) it will be 

 seen that the antennae arise at the base of the massive anther (a). As in all 



Fig. 131. Catasetum saccattim. /, the flower after removal 

 of five of the perianth leaves. //, front view of the column. ///, 

 pollinium and cement disc. /-///, after DARWIN. IV, longi- 

 tudinal section of the column ifrom nature), a, anther ; an, 

 antenna ; f, filament ; A' cement disc ; St, caudicle ; p, pollinium ; 

 T, boundary between the cement disc and rostellum ; N, stig- 

 matic cavity ; /, labellum. 



