5 14 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



objects by remaining a few hours or a day in this are wholly deprived 

 of their silica, while the tissues do not suffer. 



Preparation of Vegetable Substances. Little preparation is 

 required, beyond steeping for a short time in distilled water to get 

 rid of saline or other impurities, for mounting in preservative media 

 specimens of the minuter forms of vegetable life, or portions of the 

 larger kinds of algca, fungi, or other succulent cryptogams. But 

 the woody structures of phanerogams are often so consolidated by 

 gummy, resinous, or other deposits that sections of them should not 

 be cut until they have been softened by being partially or wholly 

 freed from these. Accordingly, pieces of stems or roots should be 

 soaked for some days in water, with the aid of a gentle heat if they 

 are very dense, and should then be steeped for some days in methy- 

 lated spirit, after which they should again be transferred to water. 

 The same treatment may be applied to hard-coated seeds, the ' stones ' 

 of fruit, ' vegetable ivory,' and other like substances. Some vegetable 

 substances, on the other hand, are too soft to be cut sufficiently thin 

 without previous hardening, either by allowing them to lose some of 

 their moisture by evaporation, or by drawing it out by steeping them 

 in spirit. Either treatment answers very well with such substances 

 as that which forms the tuber of the potato, sections of which 

 display the starch-grains in situ. Where, on the other hand, it is 

 desired to preserve colour, spirit must not be used ; and recourse nrny 

 be had to gum-imbedding, which is particularly serviceable where 

 the substance is penetrated by air-cavities, as is the case with the 

 stem of the rush, the thick leaves of the water -tity, &c. The tissue 

 is well soaked in a syrupy solution of gum arable, and this is then 

 hardened, either by allowing it to slowly evaporate, or by throwing 

 it into strong alcohol, or by freezing it. But where staining processes 

 are to be employed, the substance should be previously bleached by 

 the action of chlorine (preferably by Labarraque's chlorinated soda), 

 and then treated with alcohol for a few hours. 



For the rest, the minute structure of the higher plants is studied 

 by means of the methods of fixing, staining, and section-cutting 

 above described for the tissues of animals. For plants, absolute 

 alcohol is much used as a fixing agent, the other reagents employed 

 in their preparation being in general the same as those used in 

 animal histology. 



Staining Bacteria. It is needful to employ somewhat special- 

 ised methods for staining the saprophytic, pathogenic, and other 

 schizomycetes. Some of these stain admirably, but others, especially 

 the somewhat larger forms, are much altered, and unless observa- 

 tions are controlled with accurate and constant observations on the 

 organisms in a living condition the most egregious errors may arise. 



(1) Take half a dozen cases of putrescence in which solid tissues 

 are decomposing, but which are in different states of decomposition. 

 From each take out with a pipette a small quantity, and transfer to 

 a carefully prepared and well-filtered decoction of veal in a small 

 glass vessel, at the temperature of the respective putrefactions ; leave 

 this for half an hour. Then with a fine pipette take out a minute 

 drop from each vessel and diffuse each drop upon a cover-glass ; let 



