PRACTICAL PROCESSES IN VEGETABLE HISTOLOGY. 321 
When an organ is acted upon by a whole series of acids, bases, 
or salts, and then examined under the Microscope, it is difficult to 
disturb clearly the histological elements, and still more to pro- 
nounce upon the nature of the changes to which a given treatment 
has subjected them. Given several kind of elements, it is im- 
possible to decide what action they may exercise one upon the 
other in a mixture. Besides, all the elements being more or less 
disintegrated by the chemical treatment, we are rarely in a position 
to pronounce upon the histological nature of those which have not 
been completely dissolved. If, on the contrary, they are all 
observed in ¢he same preparation, in a thin section where they are 
only juxtaposed, all the phases of the reaction can be followed 
under the Microscope, and there is no longer any risk of being 
mistaken as to the localisation of the phenomena. 
In animal histology this method is already very advanced. In 
vegetable histology it is still very rudimentary, the sparse data 
which science possesses on the matter not having been yet collected 
into a systematic method in botanical treatises.* 
Let us first call attention to the fact that the same reagent does 
not always produce identical modifications in all those elements 
whose fundamental composition is the same. In order to produce 
the same effects in all the organs in which the elements are found 
it must often be employed in different degrees of concentration. 
Sometimes even its action must be preceded by that of another 
agent, which eliminates from the element to be discovered the 
substances masking the phenomena. It is therefore important to 
note, in the case of the majority of the reactions which will be 
indicated, in which special cases they have given good results. 
The operator ought not, moreover, to be content with a single 
reaction, ¢he accuracy of a determination resting entirely on the con- 
cordance of numerous observations. ence the many series of 
manipulations intended to render the preparations transparent, to 
fix the microscopical forms, to contract the structures, to pre- 
cipitate or dissolve certain substances, and to colour and finally to 
preserve them. 
J. CLARIFICATION. 
1. Generally the tissues are made transparent at the same time 
that thin sections are prepared. For this purpose recourse is had 
to the alkalies (ammonia or potash), to glycerin, and to chromic, 
acetic, carbolic, and nitric acids. 
*V. A. Poulsen published at Copenhagen a very excellent little book on this 
subject, translated into German (‘ Botanische Microchemie,’ Cassel, 1881) by 
C. Miiller, from which, as will be seen, we shall borrow largely. 
