i86 CONCLUDING EEMARKS AND Chap. IX. 



but obliquely towards one side where only a little 

 light entered. These results seem to imply the pre- 

 sence of some matter in the upper part which is acted 

 on by light, and which transmits its effects to the 

 lower part. It has been shown that this transmissio.i 

 is independent of the bending of the upper sensitive 

 part. We have an analogous case of transmission in 

 Drosera, for when a gland is irritated, the basal and 

 not the upper or intermediate part of the tentacle 

 bends. The flexible and sensitive filament of Dionasa 

 likewise transmits a stimulus, without itself bending ; 

 as does the stem of Mimosa. 



Light exerts a powerful influence on most vege- 

 table tissues, and there can be no doubt that it 

 generally tends to check their growth. But when the 

 two sides of a plant are illuminated in a slightly 

 different degree, it does not necessarily follow that 

 the bending towards the illuminated side is caused by 

 changes in the tissues of the same nature as those 

 which lead to increased growth in darkness. We 

 know at least that a part may bend from the light, 

 and yet its growth may not be favoured by light. 

 This is the case with the radicles of Sinapis alba, which 

 are plainly apheliotropic ; nevertheless, they grow 

 quicker in darkness than in light.* So it is with 

 many aerial roots, according to ^Viesner ;t but there 

 are other opposed cases. It appears, therefore, that 

 light does not determine the growth of apheliotropic 

 parts in any uniform manner. 



We should bear in mind that the power of bendinc 

 to the light is highly beneficial to most plants. There 



* Francis Darwin, 'Uber dna Heft Hi., 1880, p. .521. 



Waclj^tliura negativ heliotropi- t ' Sitzb. dJrk. Al;,id. derW is- 



•cher Wurzeln': ' Arbeitun des sensoli ' (Vknua), 1880, p. 12. 

 But. Inst, in Wiirzburg,' B. ii . 



