224 MOVEMENT IN PLANTS. 



intervals, and note that the curvature begins at the tip and 

 proceeds downwards ; the upper region, as it continues to 

 lean forwards, becomes straightened ; eventually (usually 

 after about six hours) the curvature is found at the base, 

 with which the upper part of the shoot forms an angle 

 of 60 to 90. 



287. Perception of Stimulus. Take a number of Oat 

 seedlings, germinated in darkness until the shoot is 2 to 

 3 cm. long. Model on the tapering point of a lead-pencil 

 a number of small conical caps of tinfoil, about 5 mm. long 

 and just large enough to fit closely over the tip of the 

 young shoot. Note that the curvature begins at the top 

 of the unshaded part, proceeding downwards more slowly 

 than when the whole seedling is illuminated, and remains 

 comparatively flat (10 to 40). This experiment should 

 be made simultaneously with the preceding one ; in both 

 cases fill the pot with soil right to the rim, to prevent any 

 shading of the shoot when placed in the chamber with light 

 entering by the lateral slit. 



Seedlings of Millet (Sorghum) and Italian Millet (Setaria 

 italica) are more sensitive than Oat seedlings, and should 

 be used for various experiments on phototropism. 



288. Perception and Transmission. Grow Oat or 

 Wheat seedlings in darkness, in pots not quite filled with 

 soil (to within, say, 2 cm. from the rim) . When the shoot 

 is about 1*5 cm. long, cover the seedling with sand or fine 

 soil so that it is almost buried, with only 2 or 3 mm. of 

 the tip projecting. Expose to one-sided illumination, and 

 note the result. No light can enter the soil (beyond a 

 depth of perhaps 2 or 3 mm.), yet the basal part of the 

 shoot curves ; evidently the heliotropic stimulus has been 

 transmitted to this part from the exposed tip. 



289. Negative Phototropism may be demonstrated 

 in the roots of some plants. Sow a few Mustard seeds on 

 muslin tied over three tumblers of water. Set the tumblers 

 in darkness : the shoots grow straight upwards, the roots 

 straight downwards. When the seedlings are well grown, 

 keep one set (A) in darkness ; put B in a position where 



