APPENDIX 235 



instruction in ecological topics during field trips primarily undertaken 

 for other purposes. So much depends on the nature of the region in 

 which the work is done, the maturity of the class, the amount of time 

 available for out-of-door study, and other circumstances, that only a few 

 general principles for field work are here suggested. 



(a) Every expedition should have one definite purpose. If many 

 incidental subjects come up, they must not interfere with the primary 

 object of the trip. 



(b) All observations must be exact. If it is desired to ascertain the 

 composition of a plant formation, the number of species and individuals 

 occurring in measured units of area should be ascertained by actual 

 count. 



Temperatures of air, soil, or water should be noted with a good 

 thermometer. 



Illumination (in shaded areas) should be measured at several periods, 

 e.g. two hours after sunrise, noon, and two hours before sunset. 



In some cases it may be practicable to determine the relative moisture 

 of the soil of several stations for at least the driest period of the year. 

 Such results should be kept on record and may be taken into account 

 by all classes which thereafter discuss the vegetation of those areas. 



(c) The distribution of vegetation forms must be studied with refer- 

 ence to extreme conditions. Xerophytic structure does not always serve 

 as a protection against a low annual rainfall but rather against severe 

 periods of drought. 



(rf) Reasoning on ecological topics should be conducted with great 

 caution. It is not safe to assume that attractiveness in any part of the 

 plant body, as in the sweet cambium layer of the white pine or the 

 inner bark of the slippery elm, is always of use to the plant. It may be 

 highly disadvantageous. On the other hand, such " defenses " as the 

 thorns on the trunk of the honey locust tree are of little or no conceiv- 

 able use (on the branches they may be advantageous). 



6. The study of Spirogyra. It is important to proceed slowly when 

 students begin to use the compound microscope, and to make sure that 

 correct habits of work are formed. The power to visualize or interpret 

 objects studied with the compound microscope should be carefully 

 trained. Simple models may be constructed. Thus the structure of the 

 cell of Spirogyra may be demonstrated very effectively with a glass jar 

 to represent the cell wall, a paper bag for the living plasma membrane, 



