CHAPTER XL 



STRUCTURE OF THE STEM OF THE LIME STUDY OF TISSUE- 

 ELEMENTS BY THE MACERATION-METHOD TYLOS.ES, OR 

 TRACHEAL PLUGS. 



PRINCIPAL MATERIALS USED. 



Twigs of Tilia europcea, three or four years old ; fresh. 



The same, outer parts of a thick stem ; in alcohol, and softened for a few 



days in half-and-half alcohol and glycerine. 

 External parts of ten to twelve year old branches of Eobinia Pseud- Acazia : 



in alcohol ; softened in glycerine-alcohol. 



PRINCIPAL REAGENTS USED. 



Iodine-glycerine Chlorzinc iodine Corallin Chlorate of potash and nitric 

 acid. 



Stem of Tilia europcea. As a further object of study, we will 

 iake the Lime tree (Tilia pamfolia, or other form of the aggre- 

 gate species, known under the name of Tilia europcea). A cross- 

 section through a twig about one-fifth of an inch in thickness 

 shows us a large-celled pith, the air-containing cells of which are 

 grouped, rosette-like, around individual narrower cells, filled with 

 finely granular brown contents. In the outer part of the pith lie 

 mucilage sacs, forming hollows in the parenchymatous tissue, 

 but which, however, are already empty. At its outermost limits 

 the pith consists of smaller cells, filled with finely granular con- 

 tents. Into this small-celled tissue project the protoxylem 

 elements of the vascular bundles. The " unwinding " spiral 

 vessels (from which the spiral thickening thread can be with- 

 drawn), are noticeable in the cross-section, from the thickening 

 bands coming out here and there. We can count about five 

 annual rings in the cross-section of a twig of one-fifth inch thick- 

 ness, and we shall perhaps notice that the successive annual 

 rings can vary greatly in thickness. In each year's ring large 



(153) 



