HELIOTROPES, OR SUNFLOWERS. 175 



proposed to designate these four movements, but for 

 our purposes we shall make as little use of them as 

 possible. 



True heliotropism prevails very extensively 

 amongst the higher plants, but there are some 

 remarkable exceptions ; as, for instance, in the " car- 

 nivorous plants." The sundews and side-saddle 

 flowers exhibit in their leaves and pitchers no trace 

 of heliotropism. The stems, tendrils, and rootlets of 

 climbing plants are often opposed to heliotropism, 

 or negatively heliotropic, whilst the leaves, on the 

 contraiy, have a general tendency to turn towards the 

 sun. Most seedlings are strongly heliotropic, even 

 though afterwards, as they grow up, they become 

 either uninfluenced by the direct light, or, in some 

 cases, turn away from it. Evidently some of the 

 lowest forms of vegetable life seek the light. 

 Strasburger says that the cells of Hcematococeus, a 

 simple unicellular alga, moved to a light which only 

 just sufficed to allow middle-sized type to be read.^ 

 It is well known how the species of Oscillai'ia con- 

 gregate towards the light. Some Desmids and 

 Diatoms exhibit the same propensity, the latter espe- 

 cially will rise to the surface and form a scum on 

 the water in the full blaze of sunlight. 



In the experiments necessary for demonstration 



' " Wirkung des Lichtes auf Schwarmporen," p. 52. 



