306 BOTANY 



PART I 



all other motoiy stimuli. Such organs become placed at right angles 

 to the briglitest rays of light to which they are exposed during their 

 development ; in the full-grown condition they assume what is known 

 as a fixed light-position. 



In too bright light the transverse position of tlie leaves becomes changed to a 

 position more or less in a line with the direction of the more intense light rays. 

 In assuming a more perpendicular position to avoid the direct rays of the mid-day 

 sun, the leaf-blades of Lactuca S'cariola and the North American SiJphium laci- 

 niatum necessarily take, according to Stahl, the direction of north and south, and 

 so are often referred to as compass plants. The foliage leaf has thus, like the 

 chloroplast of Mcsocarpus, the power of assuming either a profile or a full-face 

 position, and thus regulating tlie amount of light received. 



A number of foliage leaves possess pulvini at the base of the 

 petiole, and also at the bases of secondary and tertiary branchings ; 

 by the aid of these variation movements are effected. In this way 

 these leaves are able to change their position throughout life, and at 

 any moment to assume the position which affords them the optimal 

 supply of light. They do not have a fixed light-position determined 

 by the strongest illumination during their development, but they 

 sometimes expose their edges and sometimes their surface to the light. 



The phototropic character of organs, and depending on this the 

 specific position of the organs of the plant, may change through the 

 activity of external influences, and also at different stages of their 

 development and growth. The flower-stalks of Linaria cymhalaria 

 are at first positively phototropic. After pollination, however, they 

 become negatively phototropic, and as they elongate they push their 

 fruits into tlie crevices of the walls and rocks on which the plant 

 grows (p. 259). The intensity of the illumination has a great influence, 

 since plants which in subdued light are positively phototropic exhibit 

 negative phototropism when the illumination is excessive. Between 

 the two reactions a neutral condition exists. The adjustments to the 

 environment dependent on the intensity of light, and the search for 

 an optimal intensity of light connected with this (cf. the phototactic 

 swarm-spores, p. 301), are evidently of great use to the plant. 



Heliotropic sensibility is markedly increased when traces of coal-gas, carbon- 

 mono ,xide, etc., are present as impurities in the atmosphere. This is so strikingly 

 the case that conclusions as to the degree of impurity can be drawn from the 

 heliotropic deflection exhibited by susceptible plants (Peas, Vicia calcarata, etc.) (■'"). 



Localisation of Phototropic Perception P^). — Often the stimulus 

 of light is received at the same place that the movement is effected. 

 In certain leaves, however, the lamina is able to perceive a photo- 

 tropic stimulus without being able to carry out the corresponding 

 movement; this takes place only after the stimulus has been 

 conducted to the leaf-stalk. It is true that the leaf-stalk can also 

 react to direct stimulation, but as a rule the dominant impulse 



