19^ THE FORMATION 



purpose, solio "seconds" are used, since they are both cheap and satisfactory. 

 When an urgent demand for a finished print does arise, it is met by using 

 "velox" paper, which can be exposed in the dark room, and then developed 

 and fixed exactly like a plate. Two standard papers for views are "solio" 

 and "piatina." The former gives brown tones, and is used for contrast and 

 brilliancy, hence it is especially good for printing from negatives that have 

 too much detail and too little contrast. "Platina," on the contrary, yields 

 soft gray tones, and softens contrasts. 



FORMATION AND SUCCESSION HERRARIA 



243. Concept and purpose. A formation herbarium is a collection of 

 exsiccati, in which the species are arranged with respect to their position 

 in the formation, instead of being grouped in genera and families. Its 

 primary purpose is to furnish a record of the constitution and the structure 

 of a formation or a series of formations. At the same time, it affords the 

 basal material for developing the subject of comparative phytogeography. 

 It is impossible for one ecologist to visit many remote regions, to say noth- 

 mg of spending a period sufficient for obtaining even a fair knowledge of 

 the vegetation. He can at the best acquire an acquaintance with but few 

 regions at first hand. In consequence, a method that brings a vegetation to 

 him, with its structure carefully wrought out by years of study, is of the 

 highest value. Time, as well as distance, sets a narrow limit to the number 

 of formations which one man can investigate critically in a lifetime. It 

 !s no longer possible for a botanist to explore vast regions, and to bring 

 back results which have anything more than a very general value. This 

 fact, far from restricting the comparative study of vegetation, will serve 

 to make it more accurate and systematic. The exact results of numerous 

 resident investigators, expressed in formation herbaria, with the proper 

 series of quadrat maps and photographs, will be worked over by men who 

 are themselves specially acquainted with a particular vegetation. Compari- 

 sons will be founded upon a definite basis, and the relationship of various 

 vegetations can then be expressed in precise rather than general terms. It 

 is hardly too sweeping to assert that accurate work in the field of compara- 

 tive phytogeography can be done only in this fashion. The value of forma- 

 tion herbaria in class work is evident. On account of the limitations of 

 time and distance, classes can touch but few formations, and these at every 

 time except the growing period. For these reasons, an accurate and com- 

 plete fonnational record that can be consulted or studied at any time is 

 almost indispensable to class study in the development and structure of 

 formations. 



