216 TAGG—NOTES ON MUSEUM-METHODS 
putrefaction, arrest also, to a certain extent, intrinsic decomposi- 
tions following death. A preservative may alter also, harden, and 
render firm, or it may be render more soft, the cecssborenndl of the 
plant-tissues. 
Upon the prevention of internal changes rests the retention of » 
the natural colours, or, if these be destroyed, the prevention of the 
formation of discolouring products, while hardening, if it takes 
place before characters of form have been destroyed, fixes such 
characters permanently. 
The hardening may be due to desiccation either in the air, as 
happens when specimens are dried, or by the dehydrating action _ 
of a liquid medium, or it may result from chemical changes in 
the plant-substances induced by the preserving medium employed. 
Reviewing the results of many experiments, it is impossible 
to say of any one preserving method that it is the best; the 
choice of a method must rest not only upon a consideration of 
the general character of the specimen, but upon a consideration 
also of any special feature or character the preservation of which 
may be particularly desired. The separation of such characters 
into characters of colour and characters of form coincides with 
the separation of the methods of preserving into two groups— 
preserving by drying the specimen, and preserving by means Us 
liquid preserving media, Z 
Drying the plant has proved the only method at all satis- 
factory for preserving the colours of plants, but fails commonly 
when applied to the preservation of the natural form. Liquid 
preservatives are invaluable for the preservation of the form — 
of plants, but their use involves a sacrifice of the natural 
colours. z 
Again, as preservatives of the form of plants all liquid media 
are not equally useful, and it is necessary to distinguish those 
preserving only the form and shape of the separate parts from 
those preserving also the relationships of the parts to one 
another. Expressed concretely, the separate leaves on a twig, 
their shape, substance, and form, may be well preserved in a given 
medium, but unless there is also preserved the correct angle at 
which the leaves stand out from the stem and their relationships 
to one another in leaf-symmetry, then the prestonion of the 
form of the specimen is of a limited kind. 
