128 RELATIONS OF PLANTS TO LIGHT 



tribution of spores. It has already been pointed out that light 

 may also determine the direction in which a reaction movement 

 may be made in response to gravity (See diageotropism of 

 flowers of Narcissus). 



195. Perceptive Zones in Fhototropism. Not all of the parts 

 of the organs of the shoot are equally sensitive to the stimula- 

 tion of light. In general it is found that the apical part of a 

 stem is most sensitive, but the power of receiving the stimulus 

 is generally shared by the older parts, sometimes with an equal 

 degree of delicacy, though in other instances the power of per- 

 ception is less as the distance from the tip increases. The seed- 

 lings of the Paniceae alone show a restriction of phototropic sen- 

 sibility to the cotyledons. 



196. Localization of the Sensory Zone. Germinate a number 

 of seeds of Avena sativa (oats) in a shallow pan in a dark cham- 

 ber. When the plumule has reached a length of three cm. cover 

 the tips with a cap made of tinfoil or black paper. The caps may 

 be made by rolling squares of tinfoil around the hypocotyl like a 

 cigarette paper, and then closing one end by pinching and bend- 

 ing (Fig. 64, E). The caps should fit tightly over the tip, 

 and should cover six or seven mm. of the terminal portion. 

 Provide half a dozen plants with such coverings and set with an 

 equal number of untreated specimens in a phototropic chamber. 

 The phototropic chamber consists of a tightly made box lined 

 with black cloth, with a length of 60 cm., a width of 30 cm. 

 and a height of 30 cm. One end should be hinged and should 

 be movable like a door closing against the padded edges of the 

 box in such manner as to exclude all light. A circular opening 

 should be made in the door about eight cm ; above the bottom 

 of the box, and a shallow tin or wooden box of suitable size 

 fastened over this opening in such manner that it will hold a 

 flask with parallel walls to contain colored fluids. The tin is 

 fastened to the door by its edges, which are padded to prevent 

 the passage of light, and an opening is made in the bottom of the 

 tin box to correspond to that in the door. A hole should.be 



