f'»-Ai'. IX. SUMMARY OF CHAPTER. 489 



are apheliotropic. Some tendrils which consist of 

 modified leaves — organs in all ordinary cases strongly 

 diaheliotropic — have been rendered apheliotropic, and 

 their tips crawl into any dark crevice. 



Even in the case of ordinary heliotropic movements, 

 it is hardly credible that they result directly from 

 the action of the light, without any special adaptation. 

 We may illustrate what we mean by the hygroscopic 

 movements of plants : if the tissues on one side of an 

 organ permit of rapid evaporation, they will dry 

 quickly and contract, causing the part to bend to this 

 side. Now the wonderfully complex movements of 

 the poUinia of Orchis pyramidalis, by which they clasp 

 the proboscis of a moth and afterwards change their 

 position for the sake of depositing the pollen-masses 

 on the double stigma — or again the twisting move- 

 ments, by which certain seeds bury themselves in 

 the ground * — follow from the manner of drying of 

 the parts in question ; yet no one will suppose that 

 these results have been gained without special adapta- 

 tion. Similarly, we are led to believe in adaptation 

 when we see the hypocotyl of a seedling, which contains 

 chlorophyll, bending to the light ; for although it thus 

 receives less light, being now shaded by its own coty- 

 ledons, it places them — the more important organs — in 

 the best position to be fully illuminated. The hypo- 

 cotyl may therefore be said to sacrifice itself for the 

 good of the cotyledons, or rather of the whole plant. 

 But if it be prevented from bending, as must some- 

 times occur with seedlings springing up in an en- 

 tangled mass of vegetation, the cotyledons themselves 

 bend so as to face the light ; the one farthest off rising 



• Francis Darwin, ' On (he Hy- actions Linn. Soc.,' scries ii. vol. i 

 gr(iscopicMcdianisin,'&c., 'Trans- p. 149, 1876. 



