44 BRITISH FLOWERING PLANTS chap. 



colour intercepts the passage of the light and concen- 

 trates its effect. Darwin showed that the points of 

 growing Grass leaves were peculiarly sensitive to light, 

 but in this case no special organs have been yet 

 detected. The positions assumed by leaves are also in 

 most cases, no doubt, determined by the influence of 

 light, as Vochting has shown.^ The cells of the 

 epidermis are to some extent like the lenses of eyes. 

 They are flat below, and more or less convex externally. 

 The middle part of the cell -wall is sometimes itself 

 thickened. In certain plants some cells only of the 

 epidermis project and form lenses, thus increasing 

 the similarity to the " eyes " of some of the lower 

 animals. 



We are so accustomed to the fact that the roots of 

 plants grow downwards and the shoots upwards, that 

 it scarcely occurs to most people to ask how this is 

 efl'ected. The classical experiments of Knight called 

 attention to the subject, and showed that it is gravity 

 which enables the plant to orient itself But in this 

 case how does gravity act ? Darwin ^ showed that the 

 sensitiveness of the root is concentrated in the tip. If 

 this be removed the root cannot properly determine its 

 direction until a fresh tip has been produced. 



It was first suggested by Berthold,^ and soon after- 

 wards Haberlandt and Nemec * almost simultaneously 

 brought forward strong arguments in support of the 

 view, that this is due to the presence of movable starch 

 grains which lie free in the cells, and naturally fall to, and 

 accumulate on, the lower cell-wall. These starch grains, 

 in fact, perform the same function as the " statocysts " 

 of animals, which were formerly termed " otocysts " and 

 supposed to be auditory organs, but are now generally 

 regarded as enabling the animals to balance themselves. 

 The distribution of these free starch grains is quoted 

 in support of the theory. They occur in all organs 



1 "ijber d. Liclitstellung der Laubblatter, " Bot. Zeit. xlvi. (1888). 



^ Movements of Plants. ^ Protoplasma mechanil; 1886. 



•■ Ber. Deut. Bot. Ges. xriii. (1900). 



