STEUCTUEE OF STEMS AND EOOTS 711 



from its lower surface. The central fibre-vascular cylinder, which is 

 characteristic of the finer roots of exogens, as well as of endogens, is 

 surrounded by a single layer of cells very clearly differentiated from 

 the surrounding fundamental tissue, known as the bundle-sheath. 

 We have already seen the peculiar form assumed by the bundle- 

 sheath in the stem of ferns and other vascular cryptogams. 



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 leave 

 little to be added respecting this* special class of objects, the chief 

 points to be attended to being the preparation of the stems, &c. for 

 slicing, the sharpness 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 ite 

 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 

 prolonged 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 appropriate length 

 lias 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 revo- 

 lution, and the slice taken off with the razor, the motion given to 

 which should partake both of drawing and pushing. A little prac- 

 tice 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 cutting, but will adhere to the sur- 

 face of the razor, from which it is best detached 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 exami- 

 nation of their structure, they may be mounted either in weak 

 spirit or in glycerin-jelly. Where a mere general view only is needed, 

 dry mounting answers the purpose sufficiently well ; and there are 

 many stems, such as that of 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 sections, as it 

 usually makes the structure too transparent. Transverse sections, 

 however, when slightly charred by heating between two plates of 

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



