GERMINATION. STUDIES 57 



develop a new growth upon which the stimulus, for the most 

 part, depends. In some instances germination has been secured 

 in an infusion (implying no cooking or sterilization) of the fresh 

 tissue. 



It has been found by Eriksson that short and sudden cooling 

 has a marked influence to increase the amount of germination in 

 the aecidiospores of the wheat rust, so that a stimulus from the 

 temperature relation may be inferred. It may be well further to 

 inquire if the "resting" period is essential to the germination of 

 certain spores. If resting spores might be forced into germina- 

 tion by special stimulation, pathological work might be greatly 

 facilitated, and material made available for valuable cytological 

 studies. 



Methods of study. Studies in the germination of fungous 

 spores in solutions, or in water, are best made by the use of the 

 hanging-drop culture method generally inappropriately called the 

 Van Tieghem cell. This method consists essentially in sowing 

 the spores in a drop of the desired medium on a cover glass 

 and then inverting this cover glass over a glass ring cemented 

 to a glass slip. The old method of using slides with drop de- 

 pressions in them is not so satisfactory, and cardboard rings 

 give unreliable results. It is necessary to give the details of the 

 method referred to at considerable length. For ordinary pur- 

 poses I have found it desirable to use glass cylinders of 15-18 

 mm. internal diameter and 9-10 mm. high, preferably 16. x 10 

 mm. Xylonite rings produce products in^ the cell which may be 

 injurious. Rings of such size as indicated provide an abundance 

 of oxygen, and with them the 18 or 20 mm. square and round 

 covers are available. Round covers are preferred, since fewer acci- 

 dents occur in using them. Slips and rings must first be carefully 

 cleaned by the process previously mentioned. In some very deli- 

 cate experiments, where even the vapor from vaseline should be 

 avoided, rings may be placed in a Petri dish provided with filter 

 paper in which holes are cut for their insertion (Fig. 13,^). 



The rings are cemented to the slips by means of beeswax alone, 

 or beeswax with the addition of a small amount of vaseline. 1 For 



1 Waterproof permanent cements may also be employed, but they are not 

 generally satisfactory. 



