483 CONCLUDING HEMARKS AND CHAP. IX 



organs yot the tendency differs greatly in different 

 species, and is variable in degree in the individuals of 

 the same species, as may be seen in almost any pot 

 of seedlings of a long cultivated plant.* There is 

 therefore a basis for the modification cf this tendency 

 to almost any beneficial extent. That it has been 

 modified, we see in many cases : thus, it is of more 

 importance for insectivorous plants to place their 

 leaves in the best position for catching insects than 

 to turn their leaves to the light, and they have 

 no such power. If the stems of twining plants were 

 to bend towards the light, they would often be drawn 

 away from their supports ; and as we have seen they 

 do not thus bend. As the stems of most other plants 

 are heliotropic, we may feel almost sure that twining 

 plants, which are distributed throughout the whole 

 vascular series, have lost a power that their non- 

 climbing progenitors possessed. Moreover, with Ipo- 

 mosa, and probably all other twiners, the stem of the 

 young plant, before it begins to twine, is highly helio- 

 tropic, evidently in order to expose the cotyledons or 

 the first true leaves fully to the light. With the Ivy the 

 stems of seedlings are moderately heliotropic, whilst 

 those of the same plants when grown a little older 



* Strasburger has shown in his the light. Some individuals, more- 

 interesting work ('Wirkung des over, appear to be indifferent to 

 Lichtes . . . anf Kchwarmsporen,' the Jiglit; and those of different 

 1878), that the movement of the species behave very differently, 

 swarm-spores of various lowly The brighter the light, the 

 organised plants to a lateral light straighter is their course. They 

 is influent eil by their stage of exhibit also for a thort time the 

 development, by the temperature after-effects of light. In all tiieso 

 to which they are subjected, by respects they re.-ernble the higher 

 the degree of illumination under plants. See, also. Stahl, Ueber 

 which they have been raised, and den einfluss der Lichts auf die 

 by other unknown causes; so that Bewegungs - erscheinungen der 

 the swarm-sports of the same Schwarmsporen ' Vcrh. d. phys.- 

 species may move across the field med. Geselsshalft in Wiirzburg 

 of tho micnwo^ie cither to or from 15. xii. 1878. 



