98 



A Simple Apparatus for the Study of Phototropic 

 Responses in Seedlings. 



Geo. N. Hoffek. 



The ]iiirpose of this apparatus is to detenuiiie the miuiimim (luautity of 

 light, acting as a lateral stimulus, that will produce a ciir\ature response 

 in seedlings of various kinds as well as some of the fungi, such as 

 Phycomyccs and Piloholus. 



Any kind of light may be used, l)ut in the comparative studies I use 

 direct sunlight. The <piantity of liglit is regulated by opening and closing 

 an iris diaphragm with various-sized apertures for definite lengths of time. 



The apparatus is made from a microscope carrying case. As shown Jn 

 the photograph. Fig. 1. the outside attachments are the drawtube and rack 

 and pinion of a microscope removed troni the base and attached to one side 

 of the l)ox. The tube works through a hole in the side of the box. The 

 opening is made light proof by a velvet collar, Fig. Ill, VC, glued to the rim 

 and held to the tul)e of the secure by rubber bands. 



Into another hole is litted a hemispheric, revolving iris diaphragm. Fig. 

 Ill, I. This is on the adjacent side of the box close to the microscope and 

 en the same level with the objective of the microscope. A mirror is at- 

 tached to the box to reflect light directly into it through the iris and onto 

 the plant. A micrometer eyepiece in the microscope is the index by which 

 all of the readings are made. The illumination for the readings is supplied 

 by the light which passes through the bottle, Fig. Ill, K, into a solid glass 

 rod, SG, and conducted by the rod to within a half inch of the plane in 

 which the plant is held and ends directly opposite to the objective of the 

 microscope. This glass rod should be approximately one-half of an inch in 

 diameter so as to present a field of sufficient size. 



The bottle contains a saturated solution of bichromate of potassium In 

 water. This solution is to absorb the acti\-e blue-violet rays of light. The 

 glass rod is covered with black tape, Fig. Ill, T, and the opening into the 

 box through which the rod extends is sealed against the admission of light 



