742 SUMMARY OF CURRENT RESEARCHES RELATING TO 



ageuts on it. Note is taken of what this complex treatment elimi- 

 nates, precipitates, or colours. This result is compared with that 

 obtained by a different treatment of the same organ, or by the similar 

 treatment of a different organ, thence deducing the chemical characters 

 of the tissues experimented on. Thus pieces of wood, in which 

 the Microscope reveals the existence of cells, vessels, and fibres, no 

 longer show cells after being subjected to the influence of certain 

 substances. Another series of reagents causes them to lose their 

 fibres without destroying their vessels, whilst the former resist the 

 treatment which dissolves the walls of the vessels. 



It is to this kind of analysis that we have been so long limited. 

 The increasing perfection of practical microscopy now enables us to 

 substitute for it a more certain and productive method, that of micro- 

 chemical reactions. 



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 dis- 

 tinguish clearly the histological elements, and still more to pronounce 

 upon the nature of the changes to which a given treatment has subjected 

 them. Given several kind of elements, it is impossible 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 treat- 

 ment, we are rarely in a position to joronounce upon the histological 

 nature of those which have not been completely dissolved. If, on the 

 contrary, they are all observed in the 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 pheno- 

 mena. 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, the accuracy of a determination resting entirely on the concor~ 

 dance of numerous observations. Hence the many series of manipula- 

 tions intended to render the preparations transparent, to fix the micro- 

 scojiical forms, to contract the structures, to precipitate or dissolve 

 certain substances, and to colour and finally to preserve them. 



* V. A. Poulsen published at Copenhagen a very excellent little book on this 

 subject, trauslated into German ('Botanische Bliciochcmie,' Cassel, 1881) by 

 C. Miillcr, from which, as will be seen, we shall borrow largely. 



