48S CONCLUDING KEMAEKS AND Chap. IX 



organs — yet 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 of 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- 

 mcea, 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 



* Strnsbarger has shown iu liis the light. Some individuals, more- 



mtoresting work ('Wirkuiig des over, appear to be indifferent to 



Lichtes . . . auf Schwarmspnreii,' the light; and those of different 



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



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



cganised plants to a lateral light straighter is their course. Tliey 



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



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



to which they are subjected, by respects they resemble 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 ; su that Bewegungs - erscheinungen del 



the swarm-spores of the same Schwarmsporen ' Verh. d. phys.-. 



species may move across the field med. Geselsshalft in Wiirzburg 



of the microsa^pe cither to or from li. xii. 1878. 



