488 CONCLUDING EEMAEKS AND Chap. IX. 



organs — jvt the tendency differs greatly m 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 gro\vn a little older 



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



intei-esting work ('Wii-kung des over, appear to be indifferent to 



Lichtes . . . aufSchwarrasporeti," the light; and those of different 



1878), that tbe movonoent of the species behave very differently, 



swarm-spores of various lowly The brighter the light, th<) 



organised plants to a lateral light straishter is their course. Tliey 



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



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



to which they are subjected, by rfsp(>cts they resemble the higher 



the degree of illumination under plants. See, also, Stahl, *Uebor 



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



by other unknown causes ; so that Bewegungs - erseheinungen dei 



the swarm-spores of the same Scliwarmsporen' "Verh.°d. phys.- 



epecies may move across the field med. Geselsshalft in Wurzburg 



of the microsa^pe either to or from B. xii. 1878. 



