152 THE PLANT 



to research in experimental evolution. This is due to the fact that a knowl- 

 edge of adjustment is important in tracing the origin of new forms only 

 when adjustment is followed by adaptation, and in all such cases the ratio 

 between the two processes seems to be more or less constant. In the present 

 rudimentary development of the subject, however, it is very desirable to 

 make use of all methods of measuring functional responses to water and 

 light that are practicable in the field. Certain methods that are difficult of 

 application in nature may be used to advantage in control cultures, and the 

 results thus secured can be used to interpret those obtained from natural 

 experiments and field cultures. 



190. Method of record. As suggested elsewhere, there are four im- 

 portant kinds of records, which should be made for natural experiments, 

 and likewise for habitat and control cultures. These are exsiccati, photo- 

 graphs, biometrical formulae and curves, and histological sections. These 

 serve not merely as records of what has taken place, but they also make it 

 possible to trace the course of. evolution through a long period with an 

 accuracy otherwise impossible, and even to foreshadow the changes which 

 will occur in the future. The possibility of doing this depends primarily 

 upon the completeness of the record, and for this reason the four methods 

 indicated should be used conjointly. In the case of ecads and mutants, 

 exsiccati, photographs, and sections are the most valuable, and in the ma- 

 jority of cases are sufficient, since both ecads and mutants bear a more 

 distinctive impress than variants do. On the other hand, since variations 

 are more minute, the determination of the mean and extreme of variation 

 by biometrical methods is almost a prerequisite to the use of the other three 

 methods, which must necessarily be applied to representative individuals. 



Exsiccati and photographs are made in the usual way for plants, but it 

 is an advantage to photograph each ancestral form alongside of its proper 

 ecads, mutants, or variants, in addition to making detail pictures of each 

 form and of the organs which show modification. In the collection of 

 material for histological sections, which deal primarily with the leaf or with 

 stems in the case of plants with reduced leaves, a few simple precautions 

 have been found necessary. Whenever possible, material should be killed 

 where it is collected, since in this way the chloroplasts are fixed in their 

 normal position. In case leaves that can not be replaced easily have become 

 wilted, an immersion of 5-6 hours in water will make it possible to kill them 

 without shrinkage. In selecting leaves, great pains must be taken to collect 

 only mature leaves. When the plants have a basal rosette, or distinct 

 radical leaves, mature leaves are taken from both stem and base. In all cases 

 where the two surfaces of the leaf can not be readily distinguished, the 

 upper one is clearly marked. 



