376 THE MICROSCOPE AND ITS REVELATIONS. 



studied in the common Duckweed, every floating leaf of which has a single* 

 fibril hanging down from its lower surface. 



376. The structure of Stems and Roots cannot be thoroughly examined 

 in any other way, than by making sections in different directions with the 

 Microtome. The general instructions already given ( 184) leave little to> 

 be added respecting this special class of objects; the chief points to be 

 attended to being the preparation of the Stems, etc., for slicing, the sharp- 

 ness of the knife and the dexterity with which it is handled, and the method 

 of mounting the sections when made. The wood, if green, should first 

 be soaked in strong alcohol for a few days, to get rid of the resinous, 

 matter; and it should then be macerated in water for some days longer, 

 for the removal of its gum, before being submitted to the cutting-process. 

 If the wood be dry, it should first be softened by soaking for a sufficient, 

 length of time in water, and then treated with spirit, and afterwards with 

 water, like green wood. Some woods are so little affected even by pro- 

 longed maceration, that boiling in water is necessary to bring them to 

 the degree of softness requisite for making sections. No wood that has 

 once been dry, however, yields such good sections as that which is cut fresh. 

 When a piece, of the appropriate length, has been placed in the grasp of 

 the Section-instrument (wedges of deal or other soft wood being forced-in 

 with it, if necessary for its firm fixation), a few thick slices should first be 

 taken to reduce its surface to an exact level ; the surface should then be 

 wetted with spirit, the Micrometer-screw moved through a small part of a 

 revolution, and the slice taken off with the razor, the motion given to which 

 should partake both of drawing and pushing. A little practice will soon 

 enable the operator to discover, in each case, how thin he may venture to 

 cut his sections without a breach of continuity; and the Micrometer-screw 

 should be turned so as to give the required elevation. If the surface of 

 the wood has been sufficiently wetted, the section will not curl-up in cut- 

 ting, but will adhere to the surface of the razor, from which it is best de- 

 tached by dipping the razor in water so as to float away the slice of wood, 

 a camel-hair pencil being used to push it off, if necessary. All the sections 

 that may be found sufficiently thin and perfect, should be put aside in a 

 bottle of weak spirit until they be mounted. For the minute examination 

 of their structure, they may be mounted either in weak spirit or in gly- 

 cerine jelly. Where a mere general view only is needed, dry-mounting 

 answers the purpose sufficiently well; and there are many stems, such as the 

 Clematis, of which transverse sections rather thicker than ordinary make- 

 very beautiful opaque objects, when mounted dry on a black ground. 

 Canada Balsam should not be had recourse to, except in the case of very 

 opaque section, as it usually makes the structure too transparent. Trans- 

 verse section, however, when slightly charred by heating between two plates 

 of glass until they turn brown, may be mounted with advantage in Canada, 

 balsam and are then very showy specimens for the Gas-Microscope. The 

 number of beautiful and interesting objects which may be thus obtained 

 from even the commonest Trees, Shrubs^ and herbaceous Plants, at the 

 cost of a very small amount of trouble, can scarcely be conceived save by 

 those who have specially attended to these wonderful structures. ^ And a 

 careful study of sections made in different parts of the stem, especially in 

 the neigborhood of the ' growing point,' will reveal to the eye of the 

 Physiologist some of the most important phenomena of Vegetation. The- 

 judicious use of the staining process ( 200-203) not only improves the 

 appearance of such sections, but adds greatly to their scientific value. 

 Fossil Woods, when well preserved, are generally siUcified, and can only be. 



