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, photcgraphs, 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 variatious 
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. 
